Id agree that at the beginning, the aim is to get used to riding comfortably, for extended periods on the bike, rather than to use it as a tool for fitness. After a few weeks or months you will then be able to use your bike as a training tool for fitness , if that is what you want.
Make sure your bike fits well and is correctly adjusted.
Ride by time, not distance.
Start really easy, with 30mins, and extend the time on subsequent rides. A lot of newbies make the mistake of riding too hard and long at the start, which leads to aches, pains , injury and they view a bicycle as an instrument of torture to be endured.
Ride at a workrate which is steady, and not too hard. You should be able to speak, not be huffing and puffing. Pick a gear which enables you to spin the pedals at a high cadence (about 80 revs/min) at your chosen work-rate.

Pick your routes with care to avoid difficult or dangerous junctions. Ride your bike like vehicle (see the book/course Effective Cycling).
When you can ride a 20 mile loop at a steady pace, you are probably fit enough to join one of the slower club rides. Riding with others is a good way to learn, keep motivated and discover new routes.
Besides using your bike to "train", you can use it for all your local transport and errands. This is a great way to get extra saddle time.