>>1067302>but so long as I won't get like a fucked up spine or knee joints long term from an oversized bike,
I didn't, doesn't mean you won't. I guess I'd ride it and if it started causing soreness or feeling like a potential onset of injury then reconsider.
If you decide to look into a better/more fitting mountain bike at some point, pinkbike.com/buysell
is a great place to buy used mountain bikes. Craigslist is nice for old 70s-90s bikes, i.e. a bike for commuting, but if you want a mountain bike that will perform well on singletrack etc, you're better off with something more modern and that's better found on pinkbike.