Don't forget that the heat will take 1-2 mm out of your pace.

Your marathon time is likely to slow by three percent for every 10 degree rise in temperature from 50 F. In other words, you'll run 3 percent slower at 60 F, 6 percent slower at 70 F, 9 percent slower at 80 F, and so on. This isn't gospel; it's just a guideline
http://peakperformance.runnersworld....urns-warm.html