I figure it's due to the extreme cold. Another possibility is an issue with a joint in my big toe, right foot. It doesn't hurt all day or anything, but if I bend my toe back there's a lot of weird pressure and the kind of pain that's a bit like a stretch gone bad. Hard to explain. It doesn't bother me when I walk, but I wonder if I'm compensating in ways I don't even realize.
No matter how cold it gets, I'm still always comfortable when I'm walking. It would probably be different in wind and rain, but a plain old cold morning is just fine. Unless that's the reason my shins hurt.
Looking forward to Friday, when I can walk later and longer. Maybe I should do a route that starts out following the Monday-Thursday route and see if I get sore around the same area of the neighborhood.