Just set yourself a weekly goal and work towards that. I started off at 50 miles/week and once I was doing that consistently, I moved it up to 75 miles/week.

You can also go by time instead of distance. Set a goal of 4 hours per week and don't worry about distance. It really depends on how much time you can devote to it not how many miles you can devote. This is the best time to go out and explore routes that you can use and places you can avoid (all on your bike of course).