So sorry for the loss, as I'm sure we all are.
I think the key element here is that he was exercising in triple digit temperatures. These are dangerous temperatures even without exertion. A very small mistake on his part could put him behind the reaction curve and once you cannot think clearly, you are in real trouble.
John said it well. I have some friends that are Fighter pilots, and, besides the ridiculous watches, are taught some useful things. One of which is the OODA Loop.
In order to make a decision you have to Observe, Orient, Decide, and Act. Its important to Fighter pilots as if you can interrupt or precede the enemy in any part of this loop you have the advantage.
I know from experience, being an East Coast biker and hiker, Utah, and New Mexico, which I know better, gets inside my OODA loop. I can OBSERVE information (it is hot), but I don't ORIENT myself to it well, thus I can't DECIDE or ACT appropriately.
Every time I go into Arid and Hot, I get a local to be my buddy, just as I insist that my NM friends, when they come here, buddy up with an Easterner, they get messed up on hypothermia, gets inside their Loop.
I think this tragedy shows, we all get accustomed to the areas we frequent, and particularly when we think of ourselves as experienced. I feel like I know these Eastern Forests, and am a good outdoors man. I probably should completely loose that attitude in any survival situation. It leads to assumptions, which are basically preconceived notions that aren't linked to reality.
Thanks for sharing this difficult lesson. I'm going to try hard to be more grounded in the actual on my next trips, and not what I expect, or am used to.
best,
T