It isn't by choice. I don't ask him if he wants to, I get home from work and he either A-lifts weights with me, or B-runs. The little ones do a fairly strict regimen of push ups, sit ups, body weight squats, and several stretches.
When he hits the ball to the fence in baseball season or hits the running back so hard he nearly takes his helmet off on the football field, he isn't complaining. When he smokes his entire class in the 1 mile run, sit-ups, and push-ups in his gym class testing, he doesn't complain. When he gets 100% on his tests, he doesn't complain about having to read an hour a night either.
Success is not handed to you. It is earned.