Your body belongs to G-d, who gave it to you to care for, as a vehicle to carry your soul through life. Just as you would not harm another person, just as you would not harm anything that G-d has created, you should not harm your own body. It is your duty to eat well, to rest, to stay in shape, and to treat your body with respect in every way. Physical fitness is not arbitrary or optional; it is part of your responsibility to G-d. On the other hand, worshipping the body is destructive. The body is a vehicle for the soul; its value lies there, not as an end unto itself.
Many people today are health-conscious. We understand that being healthy makes us feel better, makes us more productive, and ultimately lengthens our lives. We realize that when we are healthy, we can concentrate on family and work, and the other things that are important to us. But most significantly, a healthy body allows you to concentrate on your soul, enabling you to fulfill your divine mission in this world and live a meaningful life.