How much and what kind of exercise you can or should do will tend to have more to do with your overall condition than you weight. Of course most overweight people are not in very good condition.
The big mistake I often made when I was fat was getting some injection of extra motivation then going after a bunch more exercise than I had been doing. Maybe that sounds good but it tends to backfire. After I wound up tired and sore or maybe even caught a cold well, guess how much I exercised after that?
The older you get the more critical it becomes to exercise just to maintain health and functional capacity.
What is important is to somehow make exercising a bigger priority. To incorporate regular exercise into your life. No matter how little you start with, the more important thing is to make it a habit. Our activities become habits over time, like over eating for example.
Personally I feel the requirement to exercise to maintain health is a design flaw. I would rather sit in front of this computer or take a nap or go sit and watch the world around me than exercise, most of the time.
I find it easier to break it up into digestible chunks. When I get up and go out and walk around for even 5-10 minutes, I have raised my metabolism to a more active state. I believe it’s more effective to do that 5 times a day than sit for 5 hours then go try to drag myself to do anything. Metabolism, every time you raise it, it stays at that level then settles back.
Ultimately you will find a 2 hour bike ride is an invigorating and relaxing experience, like I have. But, be patient and cultivate this healthy habit, the longer and heavier workouts will come easy when your body builds up to them in a natural way. It will help to remember that the next time you are getting winded and your legs are sore half way up the hill.