I find that when I ride at a higher cadence, my body tends to use more energy. On the days when I pedal slower and in a higher gear my body does not require as much fuel. As islenka suggested, maybe ride at a different intensity each day. Our metabolisms slow down as we humans age. Using 400 calories a day more than you eat should remove about a pound a week, as stated by most healthy diet sources. It seems to be a well publicized secret.

Good luck getting back in shape. Sounds like you have a great start.