I would suggest you not pay attention to how many calories you burn with exercise, for a few reasons.

First, because it is far more productive to measure calories from food intake. You can lose weight with diet and no exercise...it will be tedious, it will take a long time, and you'll probably feel like crap, but you can do it. Conversely, you cannot possibly lose weight if you exercise like a maniac but have no plan in place to limit calorie intake. The reason for this is that for any given amount of calories, it is far easier to fulfill the caloric requirement with food than it is to burn the calories with exercise. Go ahead, ride that 3-hour ride...but I bet I can nullify the calorie loss with the calories I could fit on one plate of food.

You have to go into a calorie deficit of 3500 calories -- regardless of your body weight -- to burn one pound of fat. So if you plan to ride enough to lose that in one week -- the bikecalculator.com site says that is 130-ish miles at about 16mph for a 180lb man. That's 8 hours of riding for the week. Personally I think that estimate is a little stingy; I tend to think about 40 cal/mile is closer to the truth, but that is still about 90 miles. In any case, you go out and do your 6-8 hours of riding to go into a 3500 calorie deficit. But the catch is that your body doesn't like that...it starts to get hungry and if you don't have a diet plan in place, you'll eat whatever you can get your hands on. Willpower always loses to hunger and the calories will creep into every meal and every snack. Trust me, it's easy.

You're far better off coming up with a good strategy for eating that limits calories (large volume/low calorie, eating 5 times per day, strategically using meal replacements and protein shakes, keeping a food journal) and getting a regular moderate amount of exercise. The keys are planning, developing a routine, controlling your environment and keeping records.