You know what it is? Running tempos TOO FAST. Almost every athlete and coach I've talked to about tempo runs does them too hard and fast. They think that harder is better, but it's not. Tempo runs should never be more than a moderate-moderately hard effort. You should never be straining to finish. For most, a classic 20:00-30:00 tempo run should be no less than 25 seconds/mile slower than current 5k pace. I've experimented with training on myself and I found I progressed much faster, easily, and farther when training at 80% than 100%. In just one season I dropped over 30 seconds off my 5k (from 15:32 to 14:59) when I ran SLOWER on my tempo runs. Not only did it cause me to run better, but it made running much more enjoyable knowing I just needed to run comfortably hard instead of close to all out.