Every year I have the same problem. I am training for a running race in September , it's only 7 km but very fast. The problem is I'm not fast. I've been training intensely since May (was injured from Feb to May with a hip issue). I've got my speed down from 5:20 mins to 5:05 but still not good enough. After the 4 km mark I always implode and have to slow down or go into serious oxygen debt and have to stop. Anyone got any running tips. I convinced BS is not an issue here as I can see exactly what is happening during the run on my libre, i occasionally get a small spike running but never a low. I've tried supplements i.e. iron , multi vitamin , magnesium, b12 and rhodiola tablets with no noticeable improvements.