Have you thought about more frequent, but shorter rides? I, too have knee issues related to my IT band. I know if I ride several times a week, but only 5-10 miles each ride, that my knees 'warm-up' to the idea of riding. I also try to keep my gearing light so I am literally 'spinning', as the harder I have to pump my legs the more my knees scream. I literally pushed it too hard one day, after having not ridden for a while, and had to call for a ride home. Good Luck, and don't give up yet.

Another thought - have you talked to your doc or PT about how to approach this? Not if its a good idea or not, but how to set up a training schedule to get you where you want to go, and still be able to ride in 10years.