The word picture in Philippians 3:12-14 is of men straining to win a foot race. The Christian life is especially like the longer races where the runner must sustain a winning frame of mind over a longer period of time. We cannot run our race like the hare of the "Tortoise and the Hare" fable, in which the hare took a nap during the race.
Paul illustrates that after having received God's grace, our responsibility is to return full effort to God in striving to perfection in moral, ethical, and spiritual areas. He did not see the struggle against sin, fear, and doubt as being accomplished by God alone. The apostle is here urging his erring brothers to follow his example in persistently concentrating on our common goal.
Life for us now consists of discarding wrong attitudes and habits accumulated in the past. In modern, psychological terms, we must lose our baggage. For us, the past is dead, buried in the waters of baptism. With that behind us, we must diligently make unwavering progress in putting out the leaven of sin, growing in God's love, producing the fruit of God's Spirit, moving toward the Kingdom of God, and putting on Christ's perfection, His image in us.