At this point in your training, you shouldn't base your ride on miles, but instead do it by time. How much time are you spending on the bike?. Gradually increase your training time each ride. Be carefull!! You want to make riding as fun as possible, over training will take the fun out of biking and and make you not want to ride. Baby steps at first. Then when you build up a good base, go out and kick some butt.