Three circles is right in saying that diet is 80% and exercise is 20%. It takes a pile of miles to burn a lb of fat. I figure that it takes me at least 70 miles to burn a lb of fat and that assumes that every calorie burned is fat (and it isn't) and that I do not take in as much as 1 extra calorie as a consequence of the long ride (I do eat more when I ride distances like that). So in order to lose the weight by riding, I clearly have to ride a fair bit more than 70 miles.
One problem I see is that people greatly over estimate the calorie burn of any exercise. They go out and do a 15 mile bike ride. Now the person feels that they can "reward" themselves for all the calories they burned by eating a mammoth hot fudge sundae.
If you want to lose weight, you will have to pay attention to your diet. That is all there is to it. Exercise can give you an edge but that is about it.