Your brain is part of your body. It's as simple as that.
I just celebrated 50 years of running: https://albertcory50.substack.com/p/50-years-of-running
I thought long & hard about motivation and how I've kept it up. "Will power" is not it. And I've heard every conceivable excuse about why someone does not exercise. Here's a big one: "people will look at me weird." Or on the flip side, "All my friends exercise, so I have to, too." The great Richard Feynman had something to say about that.
https://albertcory50.substack.com/p/what-do-you-care-what-ot...
And lastly, don't listen to people telling you that you need a regular schedule that you stick to, no matter what. You will never keep that up for long, and if you really hate some exercise, you'll stop:
https://albertcory50.substack.com/p/only-do-exercises-you-li...