Strong runners do struggle with the Ironman marathon. The struggle usually shows up late, after the swim and the bike have already done their work. A runner who can hold 4:00 per kilometre in an open marathon has a real advantage, but an Ironman reduces that advantage every hour the race goes on.

The run leg is 42.2 kilometres after a 3.8-kilometre swim and a 180-kilometre bike. That order changes the physiology of the race. Your legs start with cycling fatigue. Your fuel stores are partially depleted. Your core temperature is higher than it would be at the start of a road marathon. The challenge is not the distance. The challenge is the condition you're in when the distance begins.

Why the bike leaves even a strong runner starting on empty

After five to seven hours on the bike, running economy takes a hit. Running economy is the amount of oxygen you need to hold a given pace. The lower it is, the easier the pace feels. The hip flexors and quads tighten, and the lower back loses its usual extension after hours in an aero position. The first few kilometres off the bike often feel wooden because those muscles are being asked to work in a new way.

Cardiovascular drift makes this worse. During prolonged exercise, the heart rate can rise even when the pace stays the same. Fluid loss and rising core temperature are part of that. A runner who starts the Ironman marathon at a heart rate that feels normal may see her heart rate climb past threshold at the same speed by halfway through the run.

That is why a 2:50 open marathon can turn into a 3:25 Ironman run and still be a solid split. The runner hasn't lost fitness. The conditions have changed the cost of the pace.

A strong open-run engine hides Ironman-specific gaps

Road runners learn to trust their pace. In an Ironman, that sense of effort becomes less reliable. The first 10 kilometres can feel smooth because the body has just changed position and the run muscles are fresh enough. Then the glycogen drop and muscle damage combine, and the final 15 kilometres turn into a different race.

Some strong runners walk aid stations in their first Ironman and assume their run training failed. Their run training didn't fail. They simply hadn't trained the specific skill of running long off a long bike.

Fueling becomes part of the run in Ironman racing

Most long-course coaches recommend 60 to 90 grams of carbohydrate per hour on the bike, with the exact amount depending on body size and intensity. That is far more than many road marathoners practise. If your longest training runs used only water and two gels, the sustained intake of an Ironman will be unfamiliar. The gut must be trained to absorb fuel while the body is hot and moving.

TrainingPeaks' Ironman fueling guidance points to small amounts before the need arrives, tested repeatedly in long training sessions. On the run, the same rule holds: a runner who cannot process fuel late in the day will slow down, not because the legs lack strength but because the body is protecting its remaining energy.

The opening 10K is the trap

Getting off the bike feels better than expected at first. The body gets a different load, the legs stretch out, and the pace that felt enormous on the bike suddenly feels manageable. That first 10 kilometres is where strong runners lose the Ironman marathon. The wise move is to start slower than open marathon effort and keep the first half controlled.

Walking aid stations is a pacing tactic, not a failure. A 20-second walk every two or three kilometres can keep the heart rate lower and give the stomach a chance to move fluid through. Runners who refuse to walk early sometimes lose more time later, when walking is no longer a choice.

Recent results from Ironman's event hub show the same thing at the professional level. The winners run fast, but they run the final 15 kilometres with more patience than the opening 5 kilometres.

What the latest long-course races reinforced

The 2024 Ironman World Championship races in Nice and Kona gave another reminder that an Ironman marathon is a test of residual capacity. The fields were full of athletes with serious open running credentials, and the run splits still sat far behind standalone marathon times. The gap wasn't from lack of run fitness. It was the price of the previous seven hours.

That gap now shapes how coaches write Ironman training plans. Race-specific brick work and gut training have become standard in the last few seasons. Heat preparation gets added whenever a race is expected to be hot. A brick workout is a bike ride followed directly by a run. It teaches the legs and stomach to accept running after cycling.

Training the Ironman marathon if you're already a strong runner

Keep your running fitness. Add the specific durability the race demands. The key session is not the standalone long run; it's the long ride with a run off the bike. Start with ten minutes after a long ride if that feels terrible, then extend it as it becomes normal. USA Triathlon's age-group resources recommend keeping that first brick run short and controlled.

• Do one long ride with a 20- to 40-minute run off the bike every week in the last eight weeks before tapering.
• Practise taking gel or sports drink while running at goal Ironman pace during those bricks.
• Add heat training if your race is likely to be hot.
• Complete a 25- to 30K run off a four-hour ride in your race kit as a final rehearsal.

For a strong runner targeting an Ironman, a typical week might include eight to nine hours of cycling and four to five hours of running, with recovery adjusted to individual history. The long ride and the brick run should be treated as one session. What you can run after that ride is a better predictor than a fresh-leg marathon.

Before registering for a full-distance race, do a long ride followed by a 25K run. If the last 5K makes you walk, you've already found the answer. Then spend the next eight weeks fixing it before the Ironman start line.

Photo by Miguel A Amutio on Unsplash