how to train - for commuting to work

[kfergos]

I've been commuting 7 miles each way to and from work for the last year or so, and that's worked out really well for me. But that involves riding to a train, waiting at the train station, and riding from the station to work. This makes my commute 1.5 hours each way, with only 30 of the minutes actually on-bike. Plus these trains are untrustworthy and extremely schedule-limiting. However, a direct ride to my work is 20 miles, significantly longer than my normal one-way ride.

I'd like to work up to riding directly to and from work, but the three times I've made the 20-mile ride, I've had significant knee pain that has now stuck with me since then (making even the previously harmless 7-mile rides iffy). I'm betting that the huge jump from 7 miles to 20 miles caused my knee problems, and I'd like to ease into the 20-mile ride a little more. Any suggestions?

PS - I don't have a car at my disposal.