I still owe ya'll a race report, but....
It's not my achilles. Once it calmed down, I was able to figure out where the pain was coming from (as opposed to an all-over heel pain that I assumed was the achilles). I went to the running store's free injury clinic tonight, and a podiatrist informed me that it's actually the peroneal tendon, which was my best guess as well.
I feel like I know more about the lower leg anatomy than a med school student.
Anyway, he SAYS it's something he sees all the time and it's relatively fixable by adjusting some things in my shoes (heel lifts and cork to lift up my midfoot a bit) and I'll be fine for the Pig, and to keep runs to 5-7 miles for right this second. I also am not allowed to go shoeless in my house anymore, which makes me sad. I HATE SHOES.
I want to be optimistic and I'm trying to enjoy a little extra downtime (and it really is just a little, since I'm still swimming and biking) but part of me can't really be optimistic because my foot hurts. And I don't know when it won't hurt. And I don't much like it.
The good news is that I've discovered KT tape's less pretty, more durable cousin: leukotape. It holds up a million times better and seems to be making a difference.
I wonder if I'll ever have a marathon training cycle where I'll be able to actually follow a training plan without injury derailing it.