I'm pretty much in the same boat as GFish. Put some air in the old mountain bike tires and rode out and back, wasn't sure how far. I took a nap when I was done, and later drove the route to measure it. 8 miles, with one fairly long hill. Pretty much wiped me out. I kept at it, and have dropped 80 lbs over the past 4 years. I also now have a garage full of bikes, it got a bit out of control. I have no regrets at all. Lately I've been riding to work, 15 miles each way, a couple of times a week. Just enjoy yourself, I'm 59 and feel like I'm 12 when I'm on the bike!