A few weeks ago, I attended a lecture by Stu Middleman. He is a world-class, record breaking marathon runner. He actually said a lot of the way people are exercising is incorrect. They think heavy duty workouts, where you feel the burn, are heavily huffing & puffing, breathing hard, heart rate way up, is a good workout. What that actually does is put one in a sugar burning mode, rather than a fat burning mode, which is what people want to be in. He said to walk or bike at a rate just before all the really heavy breathing and exertion is a fat burning mode. One where I can still talk, and hold a conversation, but not very easily.
Once I get the bike set up, at least I know what level to aim for.