Long Distance Triathlon Pacing: Real Watts to Avoid Walking the Marathon

If you search for Ironman race pace online, you will find generic tables. Most articles and platforms tell you to aim for 65 to 75% of your FTP on the bike, and to run the marathon in the lower end of your zone 2.

It sounds mathematical and safe. The problem is that FTP measures the maximum power you can sustain for about an hour. Using a one-hour performance to predict a 5 to 7-hour metabolic effort is the main reason transition zones are full of optimistic athletes and mile 16 (kilometer 25) of the marathon is full of athletes walking.

A fixed percentage of FTP ignores your actual metabolic efficiency. One athlete might burn fat efficiently at 72% of their FTP and get off the bike ready to run. Another, at that same percentage, might be emptying their glycogen stores from mile 56 (kilometer 90).

To set your target watts, you do not need a rule of thumb. You need to measure your actual physiological limit for long efforts. You do that by looking at decoupling.

The real limit: cardiac drift

Long distance race pace is not dictated by your anaerobic threshold, but by your aerobic threshold. It is the maximum point of intensity where your body can still process the effort with a stable oxygen demand and without triggering carbohydrate burning.

The way to find this point in the real world is to measure the decoupling between your power and your heart rate (cardiac drift) during your long rides.

If you pedal at a constant 180 watts, your heart rate should stay relatively flat. If during the first hour your average pulse is 130 bpm, but in the third hour at those same 180 watts your pulse rises to 145 bpm, your body is fatiguing and the internal cost of the effort is rising. You have a decoupling above 10%.

If the decoupling is higher, that power is too high for race day, regardless of what percentage of your FTP it represents.

Bike discipline: no-go zones

An Ironman is not a bike race followed by a marathon. It is a triathlon. The way you deliver power on two wheels dictates almost entirely how you will run off the bike.

The biggest destroyer of the run is not your overall average watts, but the variability of that power. The effort spikes.

In training, short accelerations build capacity. On race day, power spikes on the bike are simply carbohydrate burners. Every time you cross your threshold to bridge up to a group or to maintain speed on a roller, you are lighting a match. You are consuming the exact glycogen you will need to maintain pace at mile 18.6 (kilometer 30) of the marathon.

This is why we set strict power caps and no-go zones on climbs. If your target power is 190 watts, your hard limit on a 6% grade should not exceed 230 watts. If that means other athletes pass you on the climb, let them go. You will see them later walking through the aid stations on the run.

The 4-week simulation

Paper calculations are validated on the road. Exactly 4 weeks before the race, we schedule a race pace simulation.

It is not the full distance, but it is a dress rehearsal of the conditions. This means executing your target watts, testing your sustained aero position, and above all, executing your nutrition at 100%. Target paces are always conditional. They assume your hydration and carbohydrate strategy goes perfectly and the weather does not present extreme conditions of heat or wind.

If you cannot ingest your planned grams of carbohydrates per hour during the simulation without gastrointestinal distress, your bike power must be adjusted downward. Less absorbed fuel means a lower metabolic limit.

Marathon execution: hunting from mile 18.6

Getting off the bike after 112 miles (180 kilometers) deceives the body. Your legs are used to a fast cadence and high speed. The first miles of the marathon always feel easier than they actually are.

Long distance run strategy relies on hard heart rate caps, not on pace per mile or kilometer.

The plan is this:

• First half (up to 13.1 miles / 21k): Conservative. Floor pace. You strictly limit yourself to your low-zone heart rate cap. If your pulse rises due to the heat, your pace drops. This is non-negotiable.
• Transition (13.1 to 18.6 miles / 21k to 30k): Maintain the effort. Your pulse will naturally start to rise (cardiac drift) even if your pace remains steady. Your job here is patience and nutrition.
• The hunt (18.6 miles / 30k to the finish): The real race starts here. If the bike was disciplined and the first half of the marathon was conservative, this is the turning point. If you have the legs, you release the heart rate cap and start looking for a negative split.

A well-executed Ironman marathon is one where you pass dozens of athletes in the final 7.5 miles (12 kilometers).

Structured plans for Long Distance

Executing this strategy requires months of preparation where volume and intensity are calibrated to push your aerobic threshold up. All our long distance plans prescribe cycling by watts and running by pace or heart rate. For US$39.99/mo, our All-Access membership includes every plan in the catalogue.

Here are the structures based on your starting point:

The counter-argument: "But I feel incredible on race day"

The most common objection to this methodology happens at mile 56 (kilometer 90) on the bike. You are halfway through the course, the weather is good, the crowds are cheering, and the target watts we set feel too easy. You think you are having an exceptional day and decide to push 15 extra watts.

The argument is that if the body responds, you should take advantage of it to bank time.

This is a basic miscalculation. Physiology is unforgiving. That extra 15 watts for two hours might save you 8 to 10 minutes on your final bike split. But the glycogen cost of that effort above your aerobic limit will catch up to you at mile 16 (kilometer 25) of the marathon.

When you run out of stores and have to start walking, your race pace goes from 5:30/km (8:51/mile) to 9:30/km (15:17/mile). In just 6.2 miles (10 kilometers) of walking, you just lost 40 minutes. You risked 40 minutes on the marathon to gain 10 on the bike.

Have a clear strategy, evaluate scenarios, and be flexible if the weather worsens. But if conditions are good and everything goes according to plan, stick to the numbers. The real race begins when you get off the bike.