I agree with the others that say keep doing what you are doing, as you seem to be losing weight, and gaining strength and endurance on the bike. Perhaps it would help you though to set some intermediate cycling goals. Often organized centuries offer other distances, so what if you shoot first to complete a 40-50 mile group ride (you can even do this in a club, not an organized ride), and then a metric century perhaps in a year, with the full century in two. I think the most important think is to listen to your body, and gradually try to increase your distance and speed. I think that is better than a formal training program that a racer might follow, at least for now.