Hi Angela
I'm in a similar situation. My training was going well until my 18 miler 3 weeks ago. I completed it but then couldn't walk the next day. After seeing several doctors and resting for two weeks (no running or anything) I went to see my friendly sports therapist. She beat up my leg, stretched and pulled me this way and that, used ultrasound and gave me some more exercises and stretches.
My problem is supra patella bursitis, an inflamed sack of fluid just above the knee cap. I've also been prescribed some stronger anti inflamitory tablets and have been doing my exercises vigerously this week. So far its feeling better and I go for a bike ride this evening and a short (5 mile) run tomorrow.
Back to the sports therapist on Tuesday. In total I've missed about 3 weeks of training at a crucial time but I am convincing myself that the base fitness will still be there and so long as I can run on Sunday pain free, I then have 2-3 weeks to do some LSR's.
Did your physio give you exercises to strengthen the appropriate muscles? If so, do those like crazy, swim lots (I should have done) and don't worry too much about the long runs. If you miss one or two thats fine. Its time on your feet and the most important thing is being fit to reach the start line. You'll be amazed at what your body can do on the day. I have a friend who never trains to more than half distance but just goes and bashes out the full marathon on the day.
Good luck and chin up.
Paul