Paul is reminding the Corinthians of what they know so he can add new information. AT: "Let me remind you that although all runners run the race, only one runner receives the prize." (See: [[rc://en/ta/man/translate/figs-rquestion]])
Paul compares living the Christian life and working for God to running a race and being an athlete. As in a race, the Christian life and work require strict discipline on the part of the runner, and, as in a race, the Christian has a specific goal. (See: [[rc://en/ta/man/translate/figs-metaphor]])
Paul is speaking of the reward God will give his faithful people as if it were a prize given for an athletic contest. (See: [[rc://en/ta/man/translate/figs-metaphor]])