I'm sorry to say this but... there is no way out of the heat (that's likely why most marathons are held in spring or fall, and not in the dead of summer). Like you, I am not very good at handling heat, but there are ways to soften the effect. If you want to get better at handling it, get used to running in the heat by training in such conditions (a little at a time).
Heat will draw your blood towards others chores than to feed your muscle (mainly to keep you cool). You can't escape that, and you'll be slower than you nornally are. But...
Run early in the morning. Eat as lightly as possible before your run. Drink plenty of water. Drink some more. Splash yourself whenever you can. Wear white. If you feel weak stop running.
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