I got my splits for the Texas Marathon last weekend. As I wrote, I slowed down in the second half, but not as much as I thought. My first half was 2:25:12 (which would have been my fifth fastest half marathon, out of seven); the second half was 2:31:45, so about a six minute decrease in speed over the second half, averaging about 27 seconds per mile.
I think this shows I can go much faster if I continue to work on my endurance. I think I'm going to do speed work one day a week and then do long miles all other days, with a really long walk on Sunday.