I understand that it is a mitzva to take good care of the body. Dear Gary, You posed an excellent question.
Exercise is certainly a mitzva when done for health reasons.
This means a person should do the utmost to preserve his health and prolong his life. In his Hilchot De'ot , Maimonides, who himself was a renowned physician, recommends exercise for preventing illness and for promoting good health. A mitzva that does not entail doing a particular deed. Well, certainly maintaining good health does not entail any specific action, such as exercising. Eating, sleeping, and looking both ways before you cross the street also help.