What I have learned from running ultras in the summer is that the only way to get conditioned for heat, is to run (or hike) in heat. The body makes a bunch of physiological adaptations to help the body perform better in the heat. Definitely need to take electrolytes for longer distances and stay well hydrated, but to truly acclimate to the heat, you need to spend some time training in the heat.
It's a lesson that gets taught to me every time I run a race where it heats up (which is a lot of them).
