Start small and work your way up. Set goals and mile markers and try no to break them. I made it up to dong 16 miles in a week on the local trails and then went for 20 on a Saturday. Then I made it 30 the next Saturday. Now I'm able to squeeze in 20 miles after work in an hour and a half. I'm going to try and get 30 done in a little over two hours.
I've had a flat since Thursday and it sucks. My new routine involves biking every day after work that it's nice and not being able to just kind of kills things. Now I kind of just sit around the house going crazy looking for a way to get out.