If the athlete has to be self-controlled and temperate in all things (food, sleep, physical training) then even more so should the Christian believer be temperate in all things and keep under, nay mortify, his body of sin (sinful flesh).
Know ye not that they which run in a race run all, but one receiveth the prize? So run, that ye may obtain. And every man that striveth for the mastery is temperate in all things. Now they do it to obtain a corruptible crown; but we an incorruptible. I therefore so run, not as uncertainly; so fight I, not as one that beateth the air: But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway (I Corinthians 9:24-27).