Not sure what I did, but yesterday, I think while I was swimming, I did something to my back. It has been pretty sore and stiff over the past day and thought I would struggle a little with my run today. It was a wet and cold, and it also rained a little on my run to work. My back did warm up and feel pretty good during the run. My only issue is that when it cools down a little, it starts to hurt.
Fortunately I have a stand up desk at work, which helps, but after a 16km run before work, I usually like sitting down a little. My run was only about 30 seconds slower than my fasted run this year, so I was also pretty happy with that.