Ironman 70.3 Training: What Is Zone 4 / Threshold?

SUMMARY:
Zone 4 is the threshold zone that sharpens race efficiency for Ironman 70.3 athletes. It targets around 87–93% of max heart rate, 91–105% of FTP and 99–104% of CSS swim speed (RPE 7–8). It feels hard but controlled, sitting right at the edge of sustainable. Training in Zone 4 improves lactate clearance, builds muscular resilience and strengthens pacing control. These sessions help you maintain strong effort deep into the swim, bike or run without fading when it matters most.

What Is Zone 4 Training?

Speed alone will not carry you through an Ironman 70.3. Endurance means little if you cannot hold your goal pace. Zone 4 training fills that gap. It is where strength, control and fitness meet to help you stay strong under pressure. Zone 4 is often called the threshold zone. It sits just below your maximum sustainable effort and feels hard but controlled. This is the point where your body is producing higher levels of lactate but can still clear it. Over time, training in Zone 4 raises that threshold, allowing you to sustain harder efforts for longer without breaking down.

Zone 4 Training Metrics:

• Heart Rate: 87–93% of Max HR

• Power (Bike): 91–105% of Functional Threshold Power (FTP)

• Effort Level: Hard but controlled, tough to sustain, but possible (RPE 7–8)

• CSS swim speed: 99–104%

• Use FLJUGA’s free calculators to find your pace, power and heart rate zones.

Unlike Zones 2 and 3, Zone 4 is not sustainable for hours, but it’s essential for sharpening speed and durability in the final 6–12 weeks before your Half Ironman.

Why Zone 4 Training Matters for Ironman 70.3

1. Raises Your Lactate Threshold

Training in Zone 4 helps you run, ride and swim faster without tiring as quickly. It builds the strength and control needed to hold a harder pace for longer, perfect for racing stronger and finishing with confidence.

2. Builds Strength & Power

Zone 4 efforts improve muscular endurance and help develop the strength needed to power through headwinds, hills and the back half of long races.

3. Trains Mental Toughness

Threshold work forces you to stay focused when things get uncomfortable. That mental grit translates directly to race-day resilience.

4. Improves Efficiency at Speed

Repeated Zone 4 sessions refine neuromuscular coordination and movement economy, helping you move faster with less effort.

When and How to Use Zone 4 Training for 70.3 Racing

Zone 4 training is a high-impact tool, but it needs to be handled with precision.

Here’s how to use it effectively:

1. Incorporate 1–2 Sessions Per Week

Threshold work is demanding. Limit it to once or twice per week to avoid burnout.

2. Focus on Controlled Intervals (3–10 Minutes)

Zone 4 isn’t about all-out sprints. Build sessions around longer, steady intervals that train you to hold race-like intensities.

3. Use It in the Build Phase (6–12 Weeks Out)

Zone 4 training is most effective during your peak prep period, when you’re converting base endurance into race-ready strength and speed.

4. Pair It with Endurance Volume

Layering Zone 4 efforts into longer workouts helps mimic race fatigue and improves your ability to hold pace when tired.

5. Prioritize Recovery

Always follow hard threshold days with Zone 1–2 recovery work or rest. This ensures adaptation, not overload.

Sample Zone 4 Workouts for Ironman 70.3 Prep

Swim: Threshold Intervals

• Warm-Up: 400 easy, 4×50 build

• Main Set: 5×200m at Zone 4 pace, 30 sec rest

• Cool-Down: 200 pull, 100 easy swim

Bike: Threshold Power Blocks

• Warm-Up: 15–20 min easy spin

• Main Set: 3×10 min @ Zone 4 FTP, 5 min recovery between

• Cool-Down: 15 min easy
  

Run: Zone 4 Tempo Intervals

• Warm-Up: 10–15 min easy jog

• Main Set: 3×8 min Zone 4 pace, 3 min jog recovery

• Cool-Down: 10 min jog + strides
  

How Zone 4 Fits Into Your 70.3 Race Prep

Zone 4 isn’t a standalone solution, it’s a sharpening tool that works best when:

• Used sparingly during the build and peak phases

• Combined with Zone 2 aerobic endurance work

• Supported by adequate recovery to avoid overtraining

By targeting this zone with precision, you’ll develop the strength and speed to hold your goal race pace when it matters most.

Red Flags: Overdoing Zone 4

Threshold training is powerful, but too much can tip you into fatigue, stagnation or injury. Zone 4 puts sustained stress on your muscular, cardiovascular and nervous systems. If you're spending too much time here without enough aerobic or recovery work, you’ll start to feel it.

Watch for these signs:

• Persistent fatigue: Feeling heavy-legged or low on energy, even after rest days

• Flattened progress: No improvement in pace, power or heart rate response

• Disrupted sleep: Trouble falling or staying asleep can signal nervous system overload

• Mood changes: Increased irritability, anxiety or loss of motivation

• Elevated resting HR: A consistently higher morning heart rate can mean your body’s under strain

Zone 4 should be used with intention , not volume. One or two sessions per week is typically enough during the build phase. If in doubt, back off slightly and refocus on Zone 2 endurance or recovery until your body responds again.

FAQ: Ironman 70.3

When should I start Zone 4 training for my 70.3?

Ideally 6–12 weeks before race day during your build phase. That’s when you’re converting endurance into race-specific fitness.

How many times a week should I do Zone 4 sessions?

Once or twice max per week. More than that can lead to fatigue without added benefit.

Is Zone 4 training the same as threshold training?

Yes. Zone 4 and threshold training both refer to training near your lactate threshold, the intensity just below redline.

FURTHER READING: BUILD YOUR IRONMAN 70.3

• Ironman 70.3 Training: What Is Zone 3 / Tempo?

• Ironman 70.3 Training: What Is Zone 4 / Threshold?

• Ironman 70.3 Training: What Is Zone 5 / VO2 Max?

• Ironman 70.3 Swim Training: 10 Threshold Sessions

• Ironman 70.3 Bike Training: 10 Threshold Sessions

• Ironman 70.3 Run Training: 10 Threshold Sessions

• Ironman 70.3 Training: Recovery Week

• Ironman 70.3: Beginner’s Guide

Final Thoughts:

Zone 4 training isn’t about going all-out at full blast or pushing yourself to the absolute limit. Instead, it’s about going strong and smart, carefully teaching your body and mind to perform efficiently just below your redline. It’s crucial to avoid overdoing it and risking burnout or injury, but at the same time, don’t skip this important intensity level, as it plays an essential role in your overall progress. Dial in your effort with clear focus and intention. Make sure to time it correctly within your broader training schedule. When done right, you’ll notice your 70.3 pace feeling smoother, more controlled and significantly stronger than ever before.

Ready to hold your pace when it counts?

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 personalised advice.