Begin with 30 minute rides on easy terrain and slowly work up to an hour over a period of about three weeks. I would ride two days then take a day off.

As you work up in time do not worry about your pace as you are just trying to adjust to the exercise. You will find your self being able to go harder as you get fitter.

If you do have a hard day where you push yourself then the next day needs to be easy for proper recovery. Ride in a gear that allows you to spin easily and just enjoy the scenery.

This is about as simple a start as I can think of and you will learn more about what you can do once you get used to your new activity.

Good luck and have fun.