My understanding is that you probably will burn more calories the longer the distance; and, that the distance traveled (or even the speed over a given distance) does not have to be a function of whether you try to be a spinner (low gears) or are a pusher (prefer low gears) over a given distance.

For example, Jan Ulrich and Lance Armstrong go the same distance in a race but they choose different gears to get there: Jan is a pusher and Lance is a spinner. The way the science has been explained to me over the years, in analyzing Lance's style and methods of training, is that it takes the same power (e.g., number of watts) to raise two bicyclists of the same weight, the same height in the same amount of time, whether the cyclists are spinning in high gears or pushing in low gears. In the '02 TDF, for instance, Lance would get to the top of L'Alpe Duez before Jan every time, even if both riders produced the same number of watts, because Lance was lighter.

So, my thought is, you try to go the distance, and you also try to work on perfecting spinning to preserve the knees. At any a given distance, you could burn up even more calories if you do the distance in less time because it will take more energy to increase the speed, everything else equal. However, you also would burn the same number of additional calories if you rode more miles with no increase in speed.

The only problem is, whether you are spinning or pushing, if you are like me, the more miles you do the hungrier you get, the more you eat, i.e., no weight loss.