It is usually one of a few things
A run that comes apart in a triathlon almost always traces back to one of these, and they leave different fingerprints in the data.
- You overbiked. The bike sat above what you could hold, and the run started fine then decayed as the legs gave out. This is the most common one.
- You under-fuelled. The wall arrives suddenly, often at a specific point, and pace collapses out of proportion to how the effort felt earlier.
- Heat. Heart rate drifts up across the day while power and pace fall, and the drop is bigger than your fitness or your bike alone would explain.
- You went out too hard on the run itself. The first kilometre or two is quick, then a long slow fade. This one is on the run, not the bike.
Why memory is a poor witness here
Over a half the whole thing is compressed into four or five hours, so the causes overlap in a way they do not on a longer day: you can be slightly short on fuel and slightly over on the bike at the same time, and the back half of the run feels identical either way. What separates them is the shape of the data, not the sensation.
The three measurements that do the separating: whether bike power faded more than 5% across the halves, whether heart-rate-to-power drift passed 10%, and whether the run decayed gradually or dropped off a cliff at one point. A gradual decay from near target pace points at the bike. A cliff points at fuel.
The fix is completely different depending on which it was, which is why guessing is expensive. More run volume will not rescue a run that failed because the bike was too hard, and better bike pacing will not rescue a fuelling plan that ran out at 14 km.
Read it from the file
Upload your race file and The Debrief reads the bike and the run together and tells you which pattern your data actually shows. In plain language, the way a coach would walk you through it, and honest about what the file can and cannot prove. If two causes are tangled together it will say that too, rather than picking one to sound confident.
A full sample debrief shows what that looks like on a real race file.