Drink lots of water, don't be ashamed to stop and rest in the shade from time to time, and sprinklers that are spraying into the street are your best friend.
Hophead has it dead on eat before you get hungry and drink before you are thirsty. If you are hungry or thirsty it is to late and it is harder to catch up in the heat.
Another thing is don't try to act like a super hero, there is nothing wrong with calling it done and heading back early. Better to take care of yourself than to get injured.
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