I guess I don't have much to contribute to the "negative split" discussion because other than marathons, I almost always run negative split. My problem is the opposite. I cannot start a 5k race fast enough, and it takes me too long to get into a decent pace. I do most runs in progression, even on easy days. So that might be the reason.
M: swimming 1000m.
T: 3 miles / 6 miles.
W: 7 miles.
R: 2km warm-up, 1000mx4 (4:40-4:39-4:39-4:38), 1 km cool down / 4 miles.
F: 7 miles.
SA: 7 miles.
SU: 14.4 miles.
The "target time" on Thursday's workout was 4:38. So I fell a bit short of that. It was very hot and humid. I was watching tennis and then soccer on Sunday morning before I went out for a run. So by the time I got out, it was another hot and humid day. After trying all kinds of gimmicks (visor/hat, sunglasses, ice bottles to cool my neck, etc.) I have finally discovered the best way to deal with heat: running more slowly. I mean, I could walk 2 hours under 90+ degrees. Then I should be able to run 2 hrs as long as it is slow enough. The key is to figure out how much I need to slow down for given circumstances.