When I bought my Mt Bike a few years back I had not ridden in quite a few years, I hopped right on and did 53 miles on a paved trail along the river then cross country all on paved roads back to the truck, some of it was up hill but not a lot. I got up the next day feeling fine, I was 48 when I did that and I have a bad back. Dirt trail is harder on your body, and your going to want to buy yourself a quality bike, learn to use the gears in your favor.