Human fat tissue contains about 87% lipids, so that 1 kg of body-fat tissue has roughly the caloric energy of 870 g of pure fat, or 7800 kcal. In principle one has to create a 7800 kcal deficit or surplus between energy intake and use to lose or gain 1 kg of body-fat. (or 3500 kcal per pound). However, if one eats 7800 kcal more than the body needs, one won't necessarily gain 1 kg of fat, since muscle and other tissues may be built. The same way, if one eats 7800 kcal less than their maintenance level, they may not lose 1 kg of fat, since muscle and sugars may be metabolized to generate energy.
Okay, so let's see, 56 pounds to lose to get to 200 pounds. That's 25.4 kg of fatty tissue or 22.1 kg of pure fat.
22.1 kg * 7800 kcal/kg = 172,364.4 kcal
172,364.4 kcal * 4.184 kJ/kcal = 721,172.65 kJ or 721 MJ.
There is approximately 131 MJ of gas in a gallon of gas or
721 MJ / 131 MJ/gal = 5.5 gal of gas around my gut, or about 150-200 miles with the Honda. I can certainly bike farther than that on the fat I've got.
 Wikipedia entry on Calorie - Trivia note #3
 Wikipedia entry on Gasoline - Energy content