The point is ,in most races (400m 800m exepted) both mentaly and phisicaly througout the race you must always feel "on the edge " but not over it.
This leads us to feel, in the comfort of the after race post mortom "if only " I had pushed on the uphill/downhill etc etc ,when the fact is ,if we had done that we would have gone into oxygen debt and bingo race over.
It is interesting to note that in some the British Milers club races one of the racers would be given his head to race "do or die" no tactics just go for it,Many died but some ran PBs .
Try it on your next race , mind you even I might pass you if you do !!