Your lungs are fine. So why do your legs fall apart after three hours?

You've done the fitness work. Your breathing is controlled, your heart rate isn't outrageous, and nothing dramatic has happened. But the pace has gone. At two hours you were running. At four hours you're negotiating with your legs.

That gap is what I coach. Fitness tells me what you can produce fresh. Durability tells me how much of it is left four hours later.

The engine isn't always the problem

The heart and lungs can hold capacity long after the running has started to fall apart. That's why the fade surprises people: they check the numbers mid-race and the numbers look survivable. Meanwhile pace at the same heart rate is falling, or the heart rate needed to hold pace is climbing. The engine still runs. Turning its output into forward motion is what gets expensive.

What actually starts going wrong

No single mechanism explains a late-race fade. Several things move at once, and they move differently in different athletes:

• Running economy deteriorates. As a long run wears on, the same pace costs more oxygen, more heart rate, more effort.
• The muscles change the deal. As fatigue and shrinking glycogen stores set in, the efficient fibres that did the early work hand over to more expensive help, and force gets harder to repeat. Pace can collapse while the cardiovascular system still has room.
• Eccentric damage accumulates. Every descent bills the quads, and the bill arrives late.
• Fuelling problems amplify all of it. Under-eating on a long day can look exactly like lost fitness in the data.

Heat, dehydration and terrain push on the same numbers, which is why I never read one signal in isolation. Drift on a hot day is mostly a hot day. Drift on a cool, flat, well-fuelled day is worth a closer look.

What I watch

Every long run gets read through the same lens:

• Pace at the same heart rate, and the heart-rate cost of holding pace.
• When in the run the deterioration begins, and how fast it goes after that.
• What the terrain was doing when it started: climbing, descending, or neither.
• Fuelling during the session, logged, not remembered.
• Whether the same pattern shows up across comparable long runs.

One run proves almost nothing. Wind, poor sleep, a warm afternoon and a stingy breakfast can all fake a durability problem, so I compare like with like: similar duration, similar terrain, similar conditions. When the pattern repeats, it earns a response. What I want to know is when the deterioration starts, how quickly it develops, and what appears to be driving it. Then I decide what to train.

Then we train the thing that failed

• Efficiency fades early even on flat ground: more sustainable easy aerobic volume.
• The fade tracks the fuelling: gut training and a proper carbohydrate strategy.
• Descents start the damage: eccentric strength and progressive downhill exposure.
• General muscular fade late on: strength work and a patient long-run progression.
• The race's specific demands are the trigger: long runs on race-like terrain, sometimes back-to-back days.
• Heat is the thread running through it: heat adaptation and a pacing plan that respects it.

None of that is an automated diagnosis. The data narrows the search; the decision about what to train, and when, is mine. The aim isn't to make every number better. It's to find the thing that stops you being the same runner late in the race, and train it until it stops being the weak link.

Evidence level: deterioration in running economy, substrate availability and muscular function during prolonged exercise is well supported. There is no single accepted laboratory measure of durability; the way I combine these signals, and what I choose to train off the back of them, is coaching application of that evidence.