1 Corinthians 9:26
ESV - 26 So I do not run aimlessly; I do not box as one beating the air.
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To paraphrase what Paul is saying; He is actual saying that his fight is not competing for vain things that will pass away but for an eternal crown of glory which remains forever in the presence of God. His competition is against the works of satin that he might be victorious in the battle for redeeming the lost and keeping himself
unspotted with the works of sin. He knows if he is to win this fight he must keep himself in shape by bringing his body into submission to will of God by exercising himself in obedience. Fasting
is part of his exercise as well.