Why Use Threshold Running in Your Marathon Training?

Summary:
Threshold running allows you to train at a high aerobic intensity while keeping the effort controlled and sustainable. Sitting around the second lactate threshold, Zone 4 challenges your ability to maintain faster running as physiological demand increases. For marathon runners, this provides another way to develop sustainable aerobic performance alongside the endurance built through easier running.

Why Is Threshold Running Useful for Marathon Training?

Marathon training requires you to develop the endurance needed to sustain running for a prolonged period, but preparing for the distance does not mean every session needs to replicate the intensity you expect to maintain on race day. Threshold running provides a way to introduce sustained work while still allowing you to accumulate meaningful time at a hard but controlled intensity. This gives you a different training stimulus from easier running and allows you to develop your ability to sustain stronger efforts without moving into the very high intensities associated with Zone 5.

Threshold running also develops a part of your aerobic fitness that may not be challenged to the same extent during easier or longer running. Working around this intensity places a substantial demand on aerobic energy production and requires you to sustain that demand for an extended period rather than producing it briefly. For marathon runners, developing this capability can contribute to your wider aerobic fitness, while providing a specific reason to include Zone 4 alongside the endurance-focused work that forms much of marathon preparation.

What Makes Zone 4 Different From Other Marathon Training Intensities?

Zone 4 sits around your second lactate threshold, commonly referred to as LT2. This represents a region of exercise intensity where lactate begins to rise more rapidly as the demand of running increases. Around LT2, lactate production is high, but the body can still clear it at a similar rate, allowing lactate concentrations to remain relatively stable for a limited period. As you move beyond LT2, the rate of lactate production increasingly exceeds the rate at which it can be removed and utilised, causing lactate concentrations to continue rising. Lactate is continually being produced and utilised during exercise, but around this intensity the balance between these processes becomes increasingly important in determining how long the effort can be sustained.

This changing metabolic demand is an important part of what separates Zone 4 from the intensities below it. Your muscles need to regenerate ATP at a greater rate to maintain the harder running effort, with aerobic metabolism continuing to provide a substantial proportion of the required energy while glycolysis operates at a greater rate. As you move further through Zone 4, relatively small increases in intensity can create a considerably greater physiological demand and reduce the duration for which the effort can be maintained. Threshold running is therefore not simply faster running, but work performed around an important physiological transition in how your body responds to increasing exercise intensity.

Where Does the Energy for Threshold Running Come From?

Your muscles require ATP to produce the contractions that allow you to run, but only a small amount of ATP is stored within the muscle itself. As running intensity increases, ATP therefore needs to be continually regenerated at a faster rate to meet the increasing energy demand. During threshold running, much of this energy continues to be supplied through aerobic metabolism, with oxygen used within the mitochondria to support the sustained production of ATP.

Carbohydrate plays an increasingly important role in providing fuel for this process. Glucose from the blood and glycogen stored within your muscles can be broken down through glycolysis, producing a small amount of ATP directly alongside pyruvate. Much of this pyruvate can then enter the mitochondria and be used for further aerobic ATP production, while pyruvate and lactate can also be converted between one another depending on the metabolic conditions within the muscle. As the rate of energy demand increases during threshold running, these processes work together to support the greater ATP requirement rather than operating as separate aerobic and anaerobic systems.

Why Is Threshold Running Still Highly Aerobic?

The harder effort associated with Zone 4 can make threshold running feel very different from easier aerobic running, but aerobic metabolism still provides a substantial proportion of the energy required. Oxygen is continually used within the mitochondria to support ATP production, with oxygen consumption increasing as your muscles require energy at a greater rate. Your cardiovascular system responds by increasing the delivery of oxygenated blood to the working muscles, while ventilation increases to support the greater exchange of oxygen and carbon dioxide.

This is important because the increase in glycolysis and lactate production around threshold does not mean your muscles have switched from aerobic to anaerobic energy production. These processes operate simultaneously, with much of the pyruvate produced through glycolysis continuing into the mitochondria where it can contribute to aerobic ATP production. Threshold running therefore places a substantial demand on your aerobic system while also requiring energy to be supplied at a faster rate, helping explain how Zone 4 can remain highly aerobic despite the greater lactate concentrations associated with the intensity.

What Happens to Your Muscles During Sustained Threshold Running?

Your muscles need to produce greater force as running intensity increases, which changes how muscle fibres are recruited to maintain the effort. Slow-twitch, Type I fibres continue to contribute substantially because of their strong capacity for aerobic energy production and resistance to fatigue. As the required force increases, additional motor units can be recruited, bringing greater involvement from Type IIa fibres. These fibres are capable of producing more force while also having a considerable capacity for aerobic metabolism, making them increasingly important during sustained threshold running.

Muscle recruitment can also change as the session continues. Fibres that have been active throughout the effort can gradually fatigue, requiring additional motor units to contribute so that the force needed to maintain running speed can still be produced. This means the muscular demand of threshold running is not determined only by how hard the opening minutes feel. Sustaining the intensity requires your muscles to continue producing the necessary force as fatigue develops, exposing a broader pool of muscle fibres to prolonged work at a substantial aerobic demand.

Can Threshold Training Increase the Pace You Can Sustain?

Threshold training is not simply about becoming more comfortable with running hard. As your fitness develops, the relationship between your running speed and the physiological demand required to produce that speed can change. Adaptations associated with aerobic training can allow you to produce more energy aerobically, while improvements in lactate transport and utilisation can contribute to maintaining the balance between lactate production and removal at greater running intensities. This can allow a faster pace to be reached before the same level of physiological demand is experienced.

For marathon runners, this matters because improvements around threshold are not limited to the pace you can maintain during Zone 4 sessions. If the speed associated with LT2 increases, running speeds below this point may represent a smaller proportion of your overall aerobic capability, although the extent of this will depend on the individual runner and how their wider fitness develops. Threshold training can therefore contribute to improving the range of sustainable running speeds available to you rather than simply teaching you to tolerate a harder effort.

Why Can Threshold Fitness Matter Over 26.2 Miles?

The physiological demands of a marathon are not identical for every runner. The intensity you can sustain across 26.2 miles depends partly on how long you are running for, meaning marathon pace may sit at a different position relative to LT2 between runners. A faster marathon runner completing the distance in considerably less time may be able to sustain a greater relative intensity than someone who remains on the course for several hours. This makes the individual relationship between threshold and marathon performance particularly important when considering how Zone 4 fits into your training.

Developing your threshold can still be relevant when your marathon intensity sits below LT2. If the speed associated with LT2 increases, the relationship between your threshold and the speeds you use elsewhere in training may also change. However, marathon performance depends on more than the position of LT2 alone. Your ability to preserve your aerobic capabilities over prolonged running influences how much of that fitness remains available as fatigue develops across 26.2 miles. Threshold fitness can therefore contribute to marathon performance, but it works alongside the endurance needed to sustain that performance over the full distance.

How Can Running Economy and Durability Support Marathon Performance?

Running economy describes the amount of oxygen your body requires to maintain a given running speed. A more economical runner can maintain the same pace at a lower oxygen cost, reducing the relative physiological demand associated with that speed. Durability describes how well you can preserve physiological characteristics as prolonged exercise continues and fatigue develops. A runner may have strong aerobic fitness and good running economy when fresh, but marathon performance also depends on how well those qualities are maintained several hours into the race.

For marathon runners, improving these qualities can influence how sustainable a given running speed becomes across 26.2 miles. Better running economy can reduce the physiological cost of maintaining pace, while greater durability can help limit the deterioration in characteristics such as economy and the relationship between pace and internal physiological demand as fatigue accumulates. Threshold training can contribute to the wider training used to develop these qualities by exposing you to sustained running under a substantial aerobic and muscular demand, although neither economy nor durability is determined by threshold training alone. Their importance comes from helping you use and preserve more of your physiological capabilities across the prolonged demands of the marathon.

What Should Zone 4 Feel Like During Marathon Training?

Zone 4 should feel hard but controlled, typically sitting around an RPE of 7–8. Your breathing will be considerably harder than during easier or tempo running and conversation will usually be limited to a few words or short phrases, such as “I’m okay, coach.” The effort should require concentration to maintain, but you should still feel in control of the intensity rather than continually increasing the pace as the session progresses. The aim is to accumulate purposeful threshold work, not to make each effort as hard as you can sustain.

Heart rate and pace can provide additional guidance, but neither should be considered in isolation. Heart rate and pace can both be influenced by terrain, heat, wind and accumulated fatigue, meaning the numbers you see during a Zone 4 session may not always reflect the same physiological demand. Heart rate can also gradually rise during sustained efforts even when your running speed remains unchanged. Using pace and heart rate alongside RPE and the talk test gives you a broader picture of the intensity you are actually experiencing. You can use the free FLJUGA Running Zone Calculators to calculate your individual running zones and use them alongside perceived effort throughout your marathon training.

How Long Should Zone 4 Work Last in Marathon Training?

There is no single duration that every marathon runner should aim for when completing Zone 4 training. The total amount of threshold work within a session may vary considerably, typically around 20–50 minutes depending on your current ability and what you want the session to achieve. More experienced runners may be able to accumulate greater amounts of controlled threshold work, while runners newer to the intensity may benefit from starting with considerably less. Your ability to recover should also influence how much threshold work you include, particularly when the session needs to fit alongside the wider demands of marathon training.

Threshold work can be completed continuously or divided into intervals, with individual efforts often lasting around 3–10 minutes. Intervals can also be considerably shorter depending on the runner and how the session is structured. Recovery periods between efforts allow you to accumulate time around the intended intensity while maintaining greater control over the session. Rather than chasing a particular duration, the aim is to manage intensity and duration so that the threshold work remains appropriate for your current ability and the wider demands of your marathon training. If you are looking for different ways to structure this type of training, explore our 10 Example Threshold Marathon Sessions for practical examples of how Zone 4 work can be incorporated into a marathon training programme.

What Happens If Your Threshold Session Becomes Too Hard?

Threshold training becomes a different type of session when the intensity repeatedly moves beyond the intended Zone 4 effort. As you run further above LT2, lactate concentrations can rise more rapidly as the rate of lactate production increasingly exceeds the rate at which it can be removed and utilised. The energetic demand also increases, making the effort progressively more difficult to sustain. Instead of accumulating controlled time around threshold, you may find that each repetition becomes shorter or requires considerably more recovery to maintain the same running speed.

This does not mean that running above threshold is inherently a problem, as higher-intensity work in Zone 5 can have its own place within marathon preparation. The issue is whether it matches the purpose of the session. If a planned threshold workout gradually becomes a much harder effort, you are changing both the physiological demand and the recovery required afterwards. Keeping Zone 4 sessions controlled allows threshold work to fulfil its intended role without unnecessarily increasing the demand it places on the rest of your marathon training. To see how Zone 4 fits into a complete preparation framework, our Marathon Running Zones Guide explains all five training zones and their distinct roles across your marathon build.

How Should Threshold Work Fit Around Your Long Runs?

Threshold running and long runs can both place substantial demands on your training week, which makes their placement worth considering rather than simply adding another hard session wherever there is space. A long run is usually designed to accumulate prolonged running and develop the endurance required for the marathon, while threshold work introduces a greater intensity over a shorter period. How closely these sessions can be placed together will depend on the runner and how demanding each session is.

The main consideration is whether one session begins to compromise the purpose of the other. If threshold work leaves you carrying substantial fatigue into a long run, you may need more recovery between the sessions or a reduction in the amount of Zone 4 work you complete. The same applies when a demanding long run affects your ability to complete threshold work with the intended control. There is no universal weekly arrangement that every marathon runner needs to follow, so placement should reflect how you recover and the wider structure of your marathon training.

Common Mistakes With Marathon Threshold Training

One common mistake is treating every threshold session as an opportunity to see how fast you can run. Threshold work should feel hard but controlled, with the aim of accumulating meaningful time around the intended intensity. Continually increasing the pace can push the session beyond Zone 4 and change the physiological demand of the workout. This often reduces the amount of appropriate threshold work you can accumulate while increasing the recovery required afterwards.

Another mistake is viewing threshold training in isolation from the rest of your marathon preparation. A well-executed Zone 4 session still creates meaningful fatigue and needs to be considered alongside your long runs and any other workouts within your week. Threshold work can provide a valuable training stimulus, but its role is to complement the wider demands of marathon training rather than dominate them. Using the intensity appropriately allows it to contribute to your development while still leaving enough recovery to train consistently.

FAQ

Can Your Threshold Change During Marathon Training?

The running speed and physiological demand associated with LT2 can change as your fitness develops. With appropriate training, you may be able to run faster before reaching a similar threshold demand, although the extent of this change will vary between runners. This is one reason your Zone 4 pace should not necessarily remain fixed throughout an entire marathon training block.

Do You Need Laboratory Testing to Find Your Threshold?

Laboratory testing can provide a more detailed assessment of your lactate response and help identify where LT2 occurs, but it is not essential for using Zone 4 effectively. Heart rate, pace, RPE and the talk test can all help you estimate and manage threshold intensity during training.

How Often Should Marathon Runners Include Threshold Training?

There is no single frequency that suits every marathon runner. How often you include threshold work should reflect your ability and the demands of the rest of your training. The session also needs enough recovery around it to avoid compromising the other work that matters during marathon preparation.

Should Every Marathon Runner Use Zone 4 Training?

Not necessarily. Threshold training can provide a useful way to develop the higher end of your aerobic fitness, but it is only one training intensity available to marathon runners. Whether you include it should depend on your current ability and how it fits within the overall structure of your training.

Final Thoughts

Threshold running gives marathon training another way to develop fitness beyond simply accumulating more distance. Used thoughtfully, Zone 4 can challenge you in ways that are difficult to achieve through easier running alone, while still remaining an intensity that can be incorporated into a structured training programme.

Marathon preparation is ultimately about developing fitness that can be carried across the full 26.2 miles. Threshold running can form part of that process without becoming the focus of the entire programme. The important consideration is whether the work supports your preparation without compromising the consistency and endurance needed for the distance.

Always consult with a medical professional or certified coach before beginning any new training program. The information provided is for educational purposes only and is not a substitute for personalized advice.