The Importance of Purity in Karma Yoga

All our hospitals in India are full-time engaged. We're always told that when you become a monk, your body now belongs to God. How can one avoid becoming too obsessed with health goals and do it in the attitude of Karma Yoga? I often think about the tension of wanting to work for optimal health versus remembering the body's temperance doesn't belong to us and therefore not really in our control. It's only when there's some amount of fitness and health, then the body is actually under our control.