The prize is the knowledge of Christ - maturity in Him. We need to learn the path is progress, not perfection. May sound like double talk. If you don't agree with me now - just wait, you will. Only let us live up to what we have already attained.
Christians have different levels of maturity and spiritual understanding.
You are responsible for what God has given you. We must not be proud of our accomplishments, yet we must not fall below the level we have attained. Needs motion to stay aloft. Called us to be disciples - followers. Paul doesn't mean that he never thought of his old life and what he had been saved from, but it means that he did not dwell on his defeats and victories of past days.