Time Differential

The last two races I've run were 20Ks--the Snowball last Sunday and the Frostbite yesterday. You can look at the time chart below and see that I was almost eight minutes faster last Sunday. I'm somewhat surprised because yesterday was much colder (about 15 degrees at the start of the race) and I usually run much better in cold weather. But the course yesterday in Forest Park is hilly, whereas the course last Sunday was completely flat. Still, I don't think the difference in terrain would make that much of a difference. I think instead I've hit a runner's slump. Or it could be that I worked too much on speed last week, and my leg's were just worn out. What else would explain dropping from 10:30/mile to 11:19/mile? You'll even notice that the better time from the Frostbite 20K last week was slower than my half marathon time on December 18, which I ran at 10:13 mile. Next week's 25K should be telling. Hopefully, I'll do better, but maybe I shouldn't train so hard throughout the coming week.