How much do you weigh? On a Target Schwinn bike, I'd be most concerned about the fork snapping or something. The cheap suspension forks on those bikes don't seem terribly sturdy. I've taken my 90' steel Miyata road bike on some mild dirt/rooty trails and it was fine on 700x25 tires with road rims. As Lester said, a MTB just does it a lot faster(without killing yourself).