I had knee pain that acted like patellar tendonitis that just wouldn't heal. My orthopedist suggested that cycling was probably the best activity that would help build/maintain strength without causing undue stress. They actually told me not to walk for exercise. Walking for my normal activities was ok, but they told me not to push it. I also went to physical therapy for a while and did a lot of strength training with ankle weights and a lot of stretching. I still do the stretching every day, actually.
With cycling, I found I needed to build up slowly in terms of mileage and always spin in low gears, but I've mostly healed and I can do what I want to do. I don't know if this will work with your particular problem, but I don't think you should give up hope of going on your trip. Maybe you should try getting your doctor to refer you for a few PT sessions. PT helped me a lot. Also, PTs will spend plenty of time with you explaining the biomechanics of your knees and why you should do some things and not others. Doctors seem to be always running to their next appointment, so you can't always get much useful info out of them.