It's called fasted training. The basic idea is to work out with a low glycogen level (often in the morning) and train the body to convert fat to usable energy. Obviously this is something that requires a progressive training plan to be feasible.

Updated answer as focus of question has changed over time.

I think you are mixing up two different things:

1. Fueling long or intense rides
2. Maintaining weight over the long term

I would not stress over trying to ingest the exact amount of calories required to fuel a ride, so that your fat reserves are not increased or diminished. Focus on what works for fueling for you to perform on the ride. You may need to take on more carbs or you may be able to use fat reserves.

Maintain weight by paying attention to what you eat for normal meals and on non-riding days. If you find you are losing weight, eat a bit more. If you are gaining weight check that you are not over-fueling during a ride, and cut back a bit on non-riding days.

Original answer:

It's called fasted training. The basic idea is to work out with a low glycogen level (often in the morning) and train the body to convert fat to usable energy. Obviously this is something that requires a progressive training plan to be feasible.