I have been racing for quite some time but have always had the same problem. For the first 5 miles or so during training rides my quads hurt like they are being beat to death, then they feel fine for 7 or 8 miles, then they start to die again for the rest of the ride. This doesn't happen during a race at all but I can't swim 1500m before every ride. The shop i go to suggested a faster spinning pace. but I've been training at 80 rpm for a month and I still have the same problem.