In reply to alastairmac:
I had a meniscal tear repaired by keyhole surgery and was winter climbing about 6 weeks later, and ran the GL3D after 3 months. Others are less fortunate of course.
I was back at the GP last month with what I thought was the return of the same problem, as the symptoms matched. But it wasn't at all, it was Patella Tendonitis (jumpers knee). It's taken about 8 weeks of rest and gentle exercise since it first started hurting but I think it's pretty much OK now.
Which is a long winded way of saying that you shouldn't self diagnose or ask advice about such things on the interweb
Knees are particularly bad for having different things go wrong with them, and the treatment for one could make a different one worse. Your best bet (easier said than done) is to find a GP or Physio with enough relevant experience to get the diagnosis right (or refer you to someone else who can).