Iliotibial band syndrome. It's quite a mouthful, isn't it? More or less, it means that a tendon along the outside of my knee has become inflamed. The good news is, in my particular case, it doesn't appear to be very serious. I have been treating it with ice and rest, and after just one week it seems to be back to normal.
The pain first started about 60 minutes into a 100 minute run. That meant that I was 40 minutes from home, so I ran through it rather than taking a long walk home… probably not a wise decision. The pain was mostly a dull ache that became sharp when I walked down stairs. The internet provided a ton of misinformation about this particular injury, but as far as I can see the causes are 1) over-training 2) bad biomechanics (angled running surfaces, bad shoes, bad running style) and 3) weak glutes resulting in poor knee stability.
So, yeah, I've gotta strengthen my buttocks for the sake of my knees. That, and my running shoes will have to be replaced because they have reached their 800km lifespan. On the plus side, this morning I did a 90 minute run that was virtually pain-free, though I do have an ice pack on my knee right now as a precaution. I could feel a bit of irritation at a couple points during the run, but to all appearances my knee hasn't suffered any damage.