I'm with Emily on the recumbent. Lower back issues are what drive a lot of folks into going with recumbents in the first place. Not every bike shop carries them, however, so you might have to do some looking. At least visit with a shop that really knows this type of bike. Not all shops do.

If you do go with a traditional MTB, one thing I would highly recommend is to go with plus size wheel setup, be it a 26+, a 27.5+, a 29+ or even a fat bike. Take it from me, someone who now only uses plus size MTBs, wider tires will do a lot to add comfort to trail riding. They're not a substitute for suspension, as some will claim, so do add that, but 3" and up tires do smooth out the trail and, especially in loose gravel, make for much less fishtailing and spooky steering. They also make for a very stable bike to ride and that's also a big plus for saving wear and tear on the back.