I found this thread late, but I had similar issues leading up to my first HM last week (Great Eastern Run in Peterborough). I was reluctant to back out, since I'd been training a lot and was very motivated, but was worried about causing more damage.
In the end I tapered more than I planned in the last fortnight, and did lots of swimming and core exercises instead of running.
I was fine on the day, but the 1 hour drive made my knee hurt again. In the end I stopped feeling any ache in my knee after a few miles and ran the whole thing. My knee hasn't hurt much this week, though that may be because my thighs are hurting so much I'm not noticing any knee pain!
I'll now take it slow and concentrate on running without pain, before I attempt another HM next spring.