I believe in moderation in all things. Well, I do now.
I’m getting on in years, so some of the idiot things I did in my 20s and 30s (and 40s and 50s) have returned to kick me in the butt.
The OP mentioned fitness. For me, that means a varied but spare diet, some focussed exercise (prescribed by my physiotherapist…), proper breaks from working - and quality sleep.
It sounds pretty mild, I know, but I find it rewarding to stick to a regime which is just uncomfortable enough to be challenging but not so brutal as to be discouraging. I mostly feel balanced, aware, and alert. And really, that’s all I want.
I find breathing exercises to be of enormous benefit, for both mind and body.