I learned last week at the doctor that I have irritated my peroneal tendon, which is what I suspected based on the location of the pain. The picture below shows the path of the peroneal tendon. Basically, the lesson here is that I pushed it too far too fast. My muscles and supporting structures needed to adjust to the extra work they were expected to do, and I didn't do it slowly enough. So -- learn from this and don't do it!!!

For the past week or so, I have been taking Advil a few times a day fo rthe inflammation, icing occasionally, and generally being easy on it. It feels pretty much normal now, and I have a follow-up visit on Monday.
The RunAMuck Mud and Music Festival is coming up this Saturday, and I still have some hopes of wearing my FiveFingers -- but I want to be wary also, because I don't want to be halfway through it and be in really bad pain. I will just have to see how I feel this week and make the best decision.
I am back to wearing my Vibram FiveFingers normally, but I haven't tried running again yet. I won't be too disappointed if I can't run in them, since I am not a runner anyway -- as long as I can still use them for walking, trail walks, biking, and anything water related!