Ran 10k for time today. My time was 55:13 which is clearly three minutes slower than the last time I ran 10k for time. Additionally, I ran 10k a couple of weeks ago at an easy pace and finished in 53:00. I felt good - not fatigued, or sick, or sore, or tight. I'm not sure why this run would be slow in comparison to other runs of the same distance. There are minor rollers on the course but that cannot be the problem - the time I ran it in 52:00 I ended with a big hill.
The one thing that I can point to that may have had a big impact is that it was very humid out. It was very foggy in some spots. I was having trouble breathing and couldn't get my HR up as high as it usually is for these time trials - my average HR was 154 vs. the 160 - 162 that I usually hit.
I double checked the distance and it was definately a 10k.