Well, I was always able to fly my VC10 to and fro where it needed to go even though I was never a jockstrapper. The only time I ever missed a flight was after injuring myself committing sport!
My theory was:
Chances of surviving the crash after being shot down - almost nil.
However,:
Likelihood of starving to death afterwards - nil (I can use my flab reserves!).
Performance of fat as an insulator in very cold water - good.
Likelihood of holding out against Spetnaz interrogation - naff all (fit or fat - no chance).
Ok, so you have to be fit enough for the body systems to hold out in flight, but as has already been said, medically fit and physically fit are two different things. War in the air is about your brain - not how well developed your pecs are.
Anyway, if you are good at sport you are never available for your squadron (ask Rory Underwood).