They didn't select for pure physical fitness but they were already selected for fitness as a pilot and then again when they were selected from the pilots to train as an astonaut. Its not like they just picked arbitrarily from the potential pool of candidates and gambled on getting better than average.

It is important to point out that prominent physicians highly recommended cigarette smoking as a beneficial hobby for all Americans to partake in.

Everybody smoked back then. Besides, until you get older your health is much more affected by your lifestyle than whether or not you smoke.