The thing I love about sports is how, even if it is a team thing, you are still an individual. I was the fat, slow, unmotivated kid. Until after college. So my body was finished developing, growing and my brain had been well nourished. I find jobs mundane, not challenging. Felt like that about med school so I stopped that investment a year in. Just went into medical research. The military was a good fit for the same reasons you mention about sports.

I chose running, the ultra marathon, distance triathlons. Even my kids like for me to keep running, biking, swimming, lifting. It keeps me learning, keeps me motivated. There is always someone better, faster, stronger. Same way I feel about piano or violin or all the homecrafting and house projects and small scale agriculture. So many tangents! I am 32, faster than ever, but it has taken more than a decade and way over 20,000 miles on foot.

PRs: ultra 50 miler in 10 hrs 32 minutes
Marathon (26.2) in 3 hrs 15 minutes
(Marathon #41)
Half marathon (13.1) in 1 hr 28 minutes
10 miler in 1 hr 7 min
10k (6.2 miles) in 43 minutes
all above in my twenties
5k while pregnant with #2! Age 31 19m54s

These have happened at different ages, stages, places.....I don't care about beating anyone but myself. It's an art.

I love sports for the mental games, the art of challenge, the challenge itself and the feeling of winning once in a while. In the case of endurance, I love the feeling of finishing, meeting your goal and setting the next one before finishing the current. It is also meditation.

You are so right on!