Bound for Disaster?

How does six months of smart training come apart without warning? I really thought I had kicked the disaster habit, but it appears that's not the case.

During the past two months I have been keenly trying to avoid stupid injuries from overtraining. I get pain - I back off - I prevent the injury - I play it smart. So, a couple days ago during a run when I started to have some (not severe) pain on the inside of my knee (toward the back), I backed off and everything seemed fine. The pain was completely gone during my brick workout on Saturday. When I started my long run on Sunday, it never occurred to me that I would be limping home. How does that happen? No warning, nothing, just severe and instant onset of pain, like someone stuck a knife in the inside of my knee. I couldn't take a step in a running gait. I stopped. I stretched. I mostly walked home. Now I'm icing and resting but I'm so confused. I've never had pain in this part of my leg - heck, I can't even figure out what muscle it is, and I certainly can't imagine being able to get a doctor's advice in a timely manner. Should I try to get through training for the next week knowing my taper is right around the corner? How much time off can I take without ruining my race day? Or is race day already in ruins? How do you learn a lesson when you didn't do anything wrong?