It's not physical energy that's needed, but rather mental energy, which has little to do with the capacities of the body. We need unwavering determination for this practice, which is transformed energy. The Buddha compared us with the man who's wearing a turban that is on fire. Obviously, if a man is wearing a turban that is on fire, he is most anxious to get rid of it. That same kind of determination is needed to practice diligently.
Energy is also dependent upon one-pointed direction.
We realize what is most important and don't vacillate between social life, social action, practice, entertainment and the many other options open to us. Everybody has more energy for those things they like. We have to be very careful that we don't use up our energy searching for pleasant sense contacts because we like them. We have to be attentive to the fact that pleasant sense contacts are so short-lived they will never give us complete satisfaction, and that we're using up our energy without getting any real fulfillment. So it turns into a waste of our energy.