Some runs are like that

My plan was to run 7 miles easy. I went out to the river trail, which is flat, and took off planning to average something like 9:15 per mile.
Well, it worked, but it was WORK. My legs felt heavy, I had no rhythm, I felt tired. This has happened before when I've tried to run at that pace. I'm more comfortable at 10 min. per mile or 8, but not at 9:15. I can't explain it.
When I got to the halfway point, I stopped for a minute to take a drink of water and catch my breath. As I took off again my only thought was to fight through it and just make it to the end.
Funny thing, though. I immediately noticed I felt much better. The jerkiness was gone out of my stride. Most of the heaviness was gone. Was there something in the water?
I did 10:08, 9:09, 9:11 and 9:25 for the first four; then 8:48, 8:26 and 8:02.
I really need to work this out, because 9 to 9:15 is about what I'd like to run the first half of my next marathon. I feel like that's the pace that prepares me best for a fast second half. I did it during my PR at Kansas City last year, so I should be able to do it again.
That's what training is all about, I guess.