I therefore so run, not as uncertainly; so fight I, not as one that beateth the air:
I - return to his main subject, his own self-denial, and his motive.
Run not as uncertainly - not without definite object as a runner uncertain of the goal. Ye gain no end in entering idol temples or eating idol meats. But I, for my part, in all acts, whether in becoming "all things to all men" or in receiving no sustenance from converts, have a definite aim to "gain the more." He who knows what to aim at, and how to aim, looks straight forward to the goal alone, casts away every encumbrance (Hebrews 12:1-2), is indifferent to what the bystanders say, and is only roused the more by even a fall.
Not as one that beateth the air - instead of beating the adversary. In the Sciamachia, or sparring in sham-fight (cf. 1 Corinthians 14:9), they struck out into the air as if at an adversary. The real adversary is Satan acting through the flesh.