Just ride until you have had enough for the day. If you are like me you will feel sore for a while, but that is a good thing. Give yourself one day off from riding per week. If you ride like this for several weeks, doing a little more mileage each week, and work on hills, you will notice a fine training effect.
With that offered as my personal opinion, be aware that I'm a beginner too. I'm trying to become a bicycling commuter. I have a head start on you, but not much of one. I have a lot to learn. I certainly am learning.
Thanks a ton
Bob