I have a "schedule" that i have stuck with for over a year now. Three work nights a week I ride a 6 mile loop around the local neighborhood. When I first started the loop would take me 45 mins. I now have the loop down to 21 mins and will be making it longer as the longer nights approach. Then, at weekends, I ride anything from 30 - 50 miles on either Sat or Sun. This will also increase as the better weather approaches.

I ride a mix of flats, rolling-hills and weather. I think its important to establish a routine that is not daunting and counter-productive to your fitness. For example, giving yourself a 20 mile nightly ride is something that is easy to talk yourself out of 3 months from now.

I would keep it simple, 3x5miles rides on weekdays and a longer one at weekends. Just until it the addiction "sticks"