I too say borrow the friends bike if it fits you well enough. Doesn't have to be perfect of course just to get out there and get hooked. Don't worry about crashing it. MTBs get crashed. No big deal. They're built to stand up to some abuse. I crashed one of my nicer road bikes gently on some wet grass and toasted the fork. It was a seriously light crash and I'm still bitter about that frail piece of crap bending so easily. I've crashed my lower end MTB pretty damn hard going over the bars after a jump and rode it away perfectly fine. I doubt anything you're going to be trying on a first ride out is going to manage to damage a MTB even if you do crash it, which you probably won't anyway. I know didn't really start crashing until I starting thinking I knew what I was doing a little bit and pushing my limits more.