I'm reading a lot into your post that you didn't state, so if I am off the mark, disregard.

I'm thinking that the doc has told you to get some exercise (or you told yourself that) to lose weight and get that heart and cardio system in shape. And that you haven't exercised for quite awhile, and you really haven't enjoyed exercising in the past???

The thing that is really important about exercising is to find something that you enjoy, and to do it in an enjoyable way. Otherwise, folks give up on their exercise. So, if you have chosen bicycling as an exercise activity, great, except don't think of it that way. Think of it as doing something fun, and make it fun.

So, in regards to your post, assuming I am correct in my assumptions (and I am probably far off the mark, and forgive me), don't push yourself too hard, as much as possible make your rides fun so you don't get discouraged and give it up.

Good luck.