I saw my doctor this morning (not a specialist, yet) about the pain in my right heel. He thinks it's achilles tendonitis, but isn't sure. He referred me to a physical therapist, who I'll be seeing twice a week for four weeks (cha-ching). I still think it could be a minor stress fracture, based on my online research of the symptoms, but I'll let the PT doc make that call.
So for now, no running. Just ibuprofen and ice. I can do gentle cycling (i.e. spinning on a trainer), but will probably opt for total rest for a few days.
Needless to say, I'm frustrated. But as a runner, it goes with the territory. I made it 17 months without a sidelining injury. Hopefully I can finally get a gait analysis, as I'm pretty sure I have some bio-mechanical problems. The wear patterns between my left and right shoes are so different that something not good has to be happening down there.
So...I'm almost positive that I'll be skipping the Lake Youngs Ultra. I'm still hopeful for the Ragnar Relay, though.
Thanks for the comments and well-wishes. I'll make sure to post updates as this unfolds.