I've finished two IMs and one marathon. In IM wisconsin '06 I had to stop running at the 17 mile mark because of knee and hip pain. When walking, no pain of that sort. When running yes. I did some research and found the suggestion that I may not have enough miles in my legs.
So this year, training for IM Lake Placid, I did the long runs every weekend from a typical marathon training program. Several long runs at 15+ and one 20 miler.
At Lake Placid, since I had some other problems going on (weird back pains that I never had before - before the race even began), I walked about half of the marathon due to back pain. So I never got to feel any knee or hip pain.
Marathon Sept. 13th. Around mile 19-20, I started having hip and knee pain. Mile 21 to the end was a voyage into pain I have never known. I was bawling my eyes out crying, I wanted to lay down and die. I kept stopping, bent over, then walking 3-5 steps and then would run (death shuffle) again. I had the goal to run the whole thing, since I had never done that before. I wondered if I was just being a wimp, since marathons do hurt. I wondered if I was doing damage to my body, yes, the pain was that bad.
I didn't care about time. I finished, and walking around had some dull soreness but not the MASSIVE PAIN I was experiencing.

I have NO knee or hip pain on any other runs, even the 20 miler... where I'll have a dull ache by the last 1/2 mile or so.

My liquid intake and nutrition was just fine during all my races. I have never ended up dehydrated nor bonking.

Any suggestions?