I drive once a year from the Phoenix metro to Carmel/Monterey for car week in August. Needless to say, there is heat. My strategy is fairly simple. I leave around 4-415AM which is getting to the coolest part of the day in the desert, which tends to be just around sunrise. I carry a cooler in the passenger footwell with ice and waters along with some yogurts. Usually throw in some gatorade.
My strategy is to stay off the interstates as much as possible, drive fast windows down and stop every 2 hours or so. My route typically takes me through the edge of the Mojave on US 95 which is a very pretty drive. Like PMax, I have my squirt bottle with fan and usually have one cooling towel on my neck and another in the cooler.
From Phoenix I can make Tehachapi or Bakersfield with little trouble. That is about 8 hours of driving or so. About all I can take.
Trying to find '68 window frames with the vent windows but they are either outrageously expensive or in really bad shape.
My advice it just embrace the environment and run as long as you feel well.
