You should do it against the clock. Say you are going to do 30 laps in 15 minutes. Then you try to do it each day a little faster. That is putting demands on the body, and that is how you build up. You keep up your energy instead of going downhill.
Jack LaLanneDon't talk age! Age has nothing to do with it. One of my guys who started out at my gym is 87 now, and he still does ten bench-press reps with a hundred-pound dumbbell in each hand. He's training to set a leg-pressing record. I put things in the guy's brain way back when, and now he'll never get away from it.
Jack LaLanne