Ok - I'm a real newbie to biking and am in the process of buying my first quality bike (ie non-wally world). My intention is to ride primarily on road and gravel, but I want to ride offroad occasionally. I generally like a MTB more than a comfort bike, but I have a lower back problem and the riding position of a MTB concerns me. I'm 32 - a little more over weight than I'd like to be, but not out of shape and have relativley minor disc problems in my lower back. So my question is will the riding position of a MTB send me back to physical therapy in a knot, or with a properly fitted bike will it be ok? I can't seem to get a consistent answers from the couple of LBS so if anyone on the board has any experiance or knoweldge on this it will be greatly appreciated.

Thanks
C.