You might try a recumbent bike. They are pricey because they aren't generally mass produced overseas like a lot of other bikes, but they are really fun to ride. And what's better for a fat guy then sitting on a lawn chair while cruising down the road??!! The long wheelbase models are easiest to ride. Underseat steering is most comfortable, and coolest!
http://www.linearrecumbent.com/linear_lwb.htm
http://www.linearrecumbent.com/linear-3-lwb-limo.htm
Just an idea, have fun and good luck with your rides and workouts.