Aerobic exercise in the heat is difficult. What I've found that works for me is "salt sticks." They're capsules of electrolytes that help rebalance during times of high water intake.

I take one after approximately consuming 1.5 liters of water, depending on temperature and water consumption rate. I also take one if my fingers start swelling; a certain sign of dehydration. It works for me and maybe it'll work for you.

This is the video that convinced me to give electrolytes a try is GearSkeptic's Performance Nutrition for Backpacking, Part 4: Electrolyte Balance. The whole playlist is good, as long as you're prepared for many hours of talking hands and reading up his sources.