I did some experimenting a couple of days ago, which involved taking it easy and being very aware of how much effort I was using. I purposely took my time and just leisurely cruised upriver against the current and the wind. After paddling all day and only a couple of times powering up for short periods, I had no pain and none the next day.
It made me realize that my average day of paddling has probably been too strenuous. I can get away with that in warmer weather, but apparently the cold and wearing a wet suit and sometimes a jacket on top of the wet suit increases the effort enough to bring on some sore muscles.
I'm going out again today and I'm going to try to keep it moderate.