If there was one thing I learned when training in Texas it was that running in the heat will prepare you for the longer races that seem to linger on into the afternoons. When taking on any distance longer than 1/2 marathon you can count on the weather changing during your race. This could be rain, snow, or heat depending on where you live and the altitudes you run.
The best way to prepare for such an impact is to train in those extremes. So, instead of waking up at 6am for your three hour run... wait until 11am, take plenty of water, and prepare for the weather change that will get you race ready for heat.