This is something I've observed personally, especially with running and mountain biking.

It had me pondering if perhaps humans evolved to become smarter in response to these frequent physical stressors. Like if we find ourselves regularly having to outrun predators and/or chasing prey to survive, it would probably be advantageous to grow smarter and depend less on endurance and speed.

So I've been taking an attitude that exercise is a faux form of brute-force survival. That the body will respond both by becoming stronger and smarter, as long as it continues experiencing having to run miles regularly to survive.