Gliding and Grinding

I run, kind of, it's pretty slow but it burns calories and helps feed that competitive side I have. Monday I ran 2.5 miles in 29 minutes and 59 seconds. I felt like I was flying, it was smooth, my breathing was great and I felt good after the run. I think that was the first time ,at least in a while that I had run that loop under 30 minutes. Today over lunch I went to run the same loop, and within the first hundred yards I was feeling a little fatigue. I thought to myself, ok, time doesn't matter let's grind this loop out and go get back to work. I did not even look at my watch to check my time at any point during the run because I knew it was going to be bad. So the end came and I looked down and it was 29:42. How did that happen? Monday I was gliding and today I was grinding, but it was faster? I think there is a moral to this story, but I am too busy to figure it out.