Runner’s World’s My Knee Aches Q&A told me this:
Patellar tendinitis is inflammation of one of the knee-joint tendons. Tendinitis occurs when tissue breakdown outpaces regrowth, and is caused by increased mileage, hills (especially downhills), and pace. Running through tendinitis will make it worse and prolong your recovery, says Davis. But treated early, it can heal in a few weeks. Here’s how: cross-train, ice, wear a patella strap, take anti-inflammatories, stretch, and do exercises to strengthen the joint and quadriceps, such as leg extensions.
I definitely need to see a doctor and not just self-diagnose, but that sounds very close to what I have. I was inconsistent with my training (which probably seems to my legs like increased mileage), and I really flew down the downhills in the SF Marathon. So until I can get to a doctor sometime this week, I’m going to try some of that advice.