Bodily fitness is the side-result, not the main result, of yoga practice

Any transcendental process of spiritual realization automatically helps one to keep the body fit, for it is the spirit soul that keeps the body always fresh. As soon as the spirit soul is out of the body, the material body immediately begins to decompose. Any spiritual process keeps the body fit without separate endeavor, but if one takes it that the ultimate aim of yoga is to maintain the body, then he is mistaken. The real perfection of yoga is elevation of the soul to a higher position or the liberation of the soul from material entanglement.

Srimad Bhagavatam 4.4.25 purport