or you could try to figure out the knee apin and work you way up (slowly) to riding the whole thing under your own power.
i bet your knee hurts cause your ramping up the mileage too fast, or pushing to high of a gear. get out there and spin! try to ride to work once a week; then twice... maybe leave your car at work one night, ride home, ride in the next morning, and then drive home...
you can ride the distance just don't try to go all out too quick.