It is usually one of a few things
A marathon that comes apart in a full-distance race almost always traces back to one of these, and they leave different fingerprints in the data.
- You overbiked. The bike sat a little above what you could truly hold for that long, and the marathon started controlled then ground down as the legs emptied. Over 140.6 even a small overbike shows up here.
- You under-fuelled. The most common full-distance cause of all. You ran a calorie and carbohydrate deficit for hours, and it caught up as a wall — pace collapsing out of proportion to how the day felt earlier.
- Heat and dehydration. Heart rate drifts up across a long 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 marathon itself. The first few kilometres are quick off the bike, then a long slow fade over the back half. This one is on the run, not the bike.
Why you cannot always tell which from memory
By the time you are walking an aid station, every one of these feels the same. Legs gone, pace gone, morale gone. The difference between them is in the shape of the data across the whole day, not in how it felt. Whether power drifted down late on the bike, whether heart rate climbed while power did not, whether the marathon decayed steadily from a fuelling deficit or fell off a cliff at one point. That is what tells you which story is true.
Getting this right matters because the fix is different for each. More run volume will not save a marathon that fell apart because the bike was too hard, and a smarter bike will not save a race you simply did not fuel. On the full distance, overbiking and under-fuelling are the two big ones and they are easily confused.
Read it from the file
Upload your race file and The Debrief reads the bike and the marathon 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 the bike and the fuelling are tangled together it will say that too, rather than picking one to sound confident.