I started biking again about a year ago (~330lbs) and I had a hand me down stiff framed MTN bike. I checked with a friend of mine who is big bike rider and everything should have been fine with my weight and bike. I took it in for a tune up too. 200 m into my very first ride I bent the rear wheel, now thats embarassing. I figured I was too heavy for a bike. I took it to a different shop to get fixed up, a new wheel with more spokes but the tech said that it bent because the person that did the tune-up didn't tighten the rear wheel correctly. So I didn't quite feel as bad.
I'm down about 35lbs now from that point, still on the same bike but I'll probably pick up a new hybrid bike in the winter.