I've never run a marathon or any such event, but did witness one in Death Valley
in August. It was a cool day ... at just 117F. Trust me, in this weather NO ONE
was peeing ... not all day despite pounding down liters of water! At those temps when running you sweat out a lot of your reserves and usually never pee!
I've heard of miracle formulas ... most of the runners I talked to were drinking plain water. Out of the 250 entrants, several were seen staggering down the road and a few collapsed in a heap in the road.
Some of the good guys at that event did the one in Hawaii as well. Also pretty warm but with humidity. All along these routes they have water stations ... I was on my bike and they gave me a couple bottles! This is where I got to chance to talk to a few of the organizers who were veterans, they felt pre hydrating was effective.
I know I did this during my 8 years racing AMA Enduro competition. Lots of water and Potassium a day or two before, bit of Carb loading too. If it works for the National Enduro Champ, then good enough for me!
Good news about riding, unless doing strenuous off road Enduro riding, riding a bike is not too stressful, even in high heat. I've ridden multiple days (5 or 6) in temps over 100F. After a week of this, you actually sort of acclimate.
You're right, lots of folks dump water over their heads. When I have easy access to more water my technique is to :
1. Wet down t-shirt and Helmet. (lasts for about an hour in 100F.
2. Suck up a nice gulp of water and let if dribble down to cool me off, or spit it out so it can blow back in my face! Very refreshing!
But when water supply runs low and you aren't sure where to top off ... you have to be very careful. This is what happened to me in Mexico ... I got lucky that time. Any time in remote areas in high heat, you have to plan ahead and conserve water, IMO.
When conserving water, I try to wait at least 5 minutes between sips ... and take only a small sip each time. 3 liters can go a LONG WAY if you do it this way.
Usually anyplace you will find Fuel, you can find Water. And that is the KEY thing about water. But sometimes those little filter straws are invaluable, IMO for true OutBack explorations.