Eventually, though, I wanted to reassure myself that it wasn't a cancerous tumor or anything, so I went a-Googling. I don't remember the exact search term I used, but it was something like "painful callus bottom of foot." That quickly pointed me toward a potential diagnosis of a corn:
Corns generally occur on the tops and sides of the toes. A hard corn is a small patch of thickened, dead skin with a packed center. A soft corn has a much thinner surface and usually occurs between the 4th and 5th toes. A seed corn is a tiny, discrete callous that can be very tender if it's on a weight-bearing part of the foot. Seed corns tend to occur on the bottom of the feet, and some doctors believe this condition is caused by plugged sweat ducts.That perfectly described what was bothering me! Further "research" provided me with suggested home remedies. After a couple of days, I went out and bought salicylic acid and corn cushions (donut-shaped adhesive pads that relieve the pressure on the corn). The salicylic acid is supposed to break down the proteins in the dead skin that makes up the corn.
The cushion definitely helped at first. The salicylic acid turned the outer layer of the corn a pasty white. I kept up with this treatment for three days. By then, the original corn cushion was getting a bit flat and hence losing some of its effectiveness.
This morning, I found a pumice stone and went to work on the corn. After soaking my foot in warm water, I rubbed gently at the corn until an outer layer of dead skin was worn down. I didn't want to irritate my foot, so I stopped there.
By dinner time, the improvement was dramatic. Mind you, I had gone for a 7-mile threshold run (5 miles at half marathon pace) in the late morning, so it's not like I had been babying my foot. Yet, I'd say that the corn is about 80% gone already.
Why didn't I treat this earlier???