Whether he believed in it or not, he passed rigorous physical tests for the Navy and NASA. They don't let just any slob be a fighter pilot, much less a test pilot or astronaut. If you don't have good cardiovascular fitness, you can't handle high G-forces or maintain good judgement while sleep deprived (those jets didn't fly themselves while the pilot napped like modern ones do).

Maybe he was just naturally fit. Some people are. But he was undoubtedly fit.