Originally Posted by
reppans
This ^. Wind force is proportional to the square of speed, so if you double your pace, you will burn 4x the energy, although for half the time. Net net, I think that works out to be twice as much calories.
Before anyone gets picky-geeky, rolling resistance is linear and, say, half of the total force at low speeds so it's technically not all of 4x the force, but this is the gist of it. It's more calories when faster.