Why wait until you're in shape to ride trails? You'll be missing the best part of mtn biking. Here we have a big group who rides (or at least starts rides) together. There are all ability levels from expert racers to true beginners and everyone is always welcome. The rides always break apart with the front guys doing the hammer-fests and there's always someone who isn't feeling "on" who rides in the back with the newbies. I'm usually somewhere in the middle unless the fast people break down and I can catch up with them. And the worse thing a beginner can do is not go, because everybody was a beginner at one time and all but the snoots understand that, and most of them are somewhere else on road bikes anyway heheh.
To answer your question, a friend of mine owns a bike shop in a college town. His biggest selling bikes are the ones in your price range, so go to a bike shop and ask questions, they will have something to fit your needs...