The first few miles of this morning's race everything seemed to be going well and then, right on schedule around mile 4, my patellar tendon started to hurt. I figured I could deal with that and pressed and and then the interior of the right knee started to hurt. Again, it was just an ache so I figured I'd ignore it, until my left ITB started to hurt. This worried me for three reasons. First, it was a new injury—I've had ITBS before, but on the other knee, and not for years. Second, my last experience with ITBS wasn't fun and you can easily get to the point where it's so painful it essentially precludes running. Third, for some reason ITBS seems especially bad when running downhill, a serious problem in an out-and-back course with 7800 feet of climbing (and thus extension). I pressed on to the 10 mile aid station where I asked one of the workers whether if I had to drop out I would be able to get a ride back to the start or whether I'd be stranded (for obvious reasons I didn't want to get further and further away if I definitely had to run back). They said I could probably get a ride back eventually though I might have to wait a while, so I pressed on.
The next 7 miles actually went pretty well. There was an easy descent and then a bunch of flat trail and the pain in both knees started to subside. It may also have helped that I got stung by some bees (again!) and this took my attention off my knee. Things seemed to be going well: I was walking the hills but doing a solid 10 min/mile and feeling very strong up till the 18 mile aid station which was in the middle of a long descent. My left knee started to hurt again on the way to the aid station and immediately afterward the pain became acute. I stretched a bit and tried to walk it off but nothing seemed to help and after a few more tentative strides it became clear I probably had to drop out. I started limping up the quarter mile walk back up to the aid station [Incidentally, people kept asking me if I was OK as I was walking back. Nice sentiment (I suspect reflexive) but I'm pretty clearly not OK. OTOH, it seemed like I was being a jerk if I said "no" since I didn't need any help. I finally settled for saying "I can make it back to the aid station.] About 100 ft away I wasn't in quite as much pain and decided maybe I should keep going. I tried running a few tentative strides uphill and the pain in my knee was so sharp it basically buckled and I almost tripped. This seemed like a good signal I should stop.
I made it up to the aid station to learn that "eventually" was indeed a while, but it turned out that another runner had twisted his ankle and had called his wife. About an hour later, she came to pick him up and I hitched a ride back to the start and headed home. I spent that hour walking around and never got past the point where the range of pain-free motion in my knee was more than about 45 degrees, so it seems pretty likely it would have gotten a lot worse had I chosen to keep going. Even now, I only have about 100 degrees before it's painful and that's probably because I've been sitting around rather than running. That said, it's always a bummer to DNF, and after the usual rest/ice rehab, I expect to make a try at another 50 miler.

Yeah, I know. It sucks getting old. Parts wear out. 50 milers tend to accelerate the process. PeeWee Golf, man. You can safely play well into your 90s. $.02