THis is the forum you are seeking. Many of the regulars here started out just as you describe and have moved on to ever more demanding rides and expensive wheels. Others are just getting started. Yet others just pedal along at whatever pace suits them.

If you want to use riding to improve your fitness, the only advice we have is keep at it. Don't worry so much about "progress" and don't get discouraged if your progress is slow (or at times, even goes backwards). Just find a way to ride that you enjoy. Then repeat.