Welcome to singletracks! Your story reminds me of when I was learning to ride a bike and whatnot, I never had a very nice bike, and for years I didn’t have a bike that was for my gender. (I was stuck with girls’ bikes for years!) 😳
Mountain biking is a great way to get in shape, is easier on the joints than running, and is also a great way to sightsee.
A small tidbit of advice that I suppose goes without saying, but I’ll say it anyway. You must be confident with yourself and your bike when riding on trails (or anywhere for that matter), make sure you know your limitations and don’t become cocky and overconfident!
Good luck with your newfound hobby!