For me it's as simple as this: ride=lose weight. don't ride=gain weight.
I keep flitting back and forth between "Normal" and "Overweight" depending on my ability to get out on the bike (normal constraints of life and weather).
In any case, since my back injury last Fall I have climbed back up to a degree of physical fitness very much like what I had before: Did a 51.5 mile ride last week and the other day maintained an average speed of 14.4 mph for 25 miles. Maybe I'll try that Century some day soon; that would be something; first 100 mile day at 77!