I second what emac65 says.
-Make sure that whatever you do, you enjoy it. Getting weight off and keeping it off is all about changing your lifestyle permanently, even if it’s only a small change. If you start out exercising like crazy and not enjoying it, you will probably give up sooner or later. Make sure you stay within your limits, and build up how much you do over time.
-I would say that it is also helpful to get a routine going – e.g. cycle the same days each week. If you exercise at random unplanned times you are more likely to forget/skip/give up in the end.
-Also, keep an eye on your performance, e.g. time yourself, so that you know when you get better. But don’t get down if it takes a while either! That way you can compete against yourself.
-Finally, if you can get a riding buddy of a similar fitness and incliniation to get fitter, you can organise regular rides and so will be more motivated to keep it going. Less chance of not bothering because its a bit damp/tired/footie is on etc.