Walk very far distances. Before cars, a lot of people walked between towns. It was completely normal and safe if you know what you're doing. I have friends who sometimes walk up to twenty miles to the neighboring towns because they swore off vehicles, and they show up and people are flabbergasted like they're thinking "holy crap it's a miracle you must be gods" as if the friends didn't just have to use two feet for two hours to get there. But that's what a human is supposed to do, biologically that is. And you don't even have to run, in fact it's better for your body and mind to walk than run.

Also, sleep well and stay hydrated. Sleep controls metabolism. Metabolism controls fitness. Bad sleep equals bad metabolism. Same goes for hydration. Drink a pint of water every time you piss. That's probably why there are drinking fountains next to bathrooms. All this advice has worked for me too.