When I started keeping track of my pace way back last year sometime, I learned that I keep a pretty steady pace around the 12min/mile point, give or take a bit depending on all sorts of things.
Today, if my calculations are right, I ran the first half of my run at a 20 minute mile pace (!!!), and the second half at 10:30. Ooookay... That's weird enough, but on top of that, I can't figure out why the halves didn't feel at all different.
(Actually, I feel like this isn't possibly true. I normally walk at 20min/mile or slightly more, and I was passing walkers at a reasonable rate both ways this morning. Maybe I just need a new stopwatch.)