So I'm trotting along on my 18-mile long run yesterday. Run, run, happy, happy. It's the first time I've done a long run outside (with some large unidentifiable yellow orb in the sky) in over a month. I'm thrilled to be back. 

Chatting with friends, I mentally make note of a light twinge behind my right kneecap. It's not pain. More like a warning that there could be pain. Big deal. It's not a run unless something hurts. 

Since most people I run with aren't training for a marathon (or they are but are considerably faster than  me), they drift off after mile 10. I keep running. At mile 12, I stop for a water break. Throwing down the water bottle, I stand back up, hit the "Start" button on my watch... and almost hit the ground as my right knee buckles beneath me.