Here is an inspirational story about Englishman Gary Brennan, who at the beginning of 2008 weighed 546 lbs.
He started riding a bike 7 miles each way to work, five days a week and in less that a year lost 224 lbs.
The top picture shows Gary as he was, and the picture on the left is as he is now.
Doctors had previously told him the only way he could lose the weight was to have gastric bipass surgury.
When he started riding to work the seven mile ride took him two hours. I cannot imagine the effort it must have taken to haul over 500 lbs on a bike, up even the slightest incline. Today the daily commute takes Gary about twenty minutes each way.
Cycling is a great way to burn calories. It can be as gentle or intense as you wish to make it. I can imagine when Gary started out just the amount of weight he had to propel forward burned a lot of calories even at a slow speed.