It's a lightly pulled muscle. It's not bad, but I feel it when I walk. No jogging for a bit. koschei23 was right, I was overdoing it.
Woke up this morning just before 7:00 AM. I tried, unsuccessfully, to get back to sleep when I finally said "screw it" and got out of bed. After much piddling around, I pulled my bike out and went riding for over an hour. There's no strain on the calf and biking is something my body is quite used to. I probably could have ridden for another hour, had I not been stupid and forgotten a water bottle.