McEnroe was trained by Harry Hopman in college. He was noticably much bigger and stronger in his early years on the tour:
Much more diciplined in how he played too. He was hitting his groundstrokes bigger than Vilas here, out Vilasing Vilas!
edit: Guess I was wrong about Harry Hopman coaching him in college, it was actually earlier. but I guess his having a scholarship in college meant he had to do what he was told and whatever physical programme they had him in college had a great effect on him. Maybe he would have been even better if he'd kept up his focus on fitness.