The ancient Greeks had a saying: "A sound mind in a sound body." Physical and mental health necessarily go together.
Exercise strengthens. Both the body and the mind need exercise to keep fit. The two are necessarily exercise partners, the mind encouraging the body to exercise to maintain itself, the body providing a convenient venue for the mind to exercise in.
I get lots of mental exercise, and my mind is strong and healthy.
My body, however, has been coddled and ignored. My mind knows it has been neglectful in its job of being the Exercise Buddy for my body, and apologizes. It is determined to make amends.
A mind stuck within a body unused to physical exercise cannot easily be coaxed into pretending that initiating engagement with lax muscles will be either fun or interesting.
PRO TIP
For best results in starting a brand-new exercise routine, you must trick the sloth mind into an unwitting cooperation. Confuse it with caffeine and stimulate it with dance music. The mind will never know what hit it. The body will thank you.