After ten or fifteen miles, my right shoulder often hurts. I didn't think much about this for a long time, since it's not unusual for my shoulders to hurt a bit, but the recurring pain finally bothered me enough to assign a few brain cells to the problem. Their report: it's a combination of being right-handed and giving a big friendly wave to those courteous motorists who stop for me at unregulated crossings. It's an awkward wave because of the need to balance, stride, and face forward as I do it.
I'm testing this diagnosis by waving left-handed for now. After two days, I think it's helping. Once my right shoulder recovers, the long-term solution might be to wave left with the right hand and right with the left hand, so my arms stay close to my body and there's less strain on the shoulders.
Today: 13 miles. This year: 853 miles.