Right, and everything I have found publicly available about these trails, leads me to believe they're not the type of place an experienced person would get lost. At least not in a serious way. PK has solid navigation skills, and solid survival skills.
Let's talk about his not being "heat adapted". As a former president of an outing club, he would 100% have knowledge and practice with being outdoors in all kinds of conditions. This is just a casual Saturday morning run, so what motivation would he have to push himself into heat stroke?
These are the reasons I am leaning towards either a catastrophic medical emergency, like a sudden heart attack where he somehow dragged himself off the trail and has been tragically overlooked, or he parked the car and never ran the trail.
I have no explanation for why he would do the latter.
Sadly, one of the symptoms of heat stroke in most people is...euphoria. Not only have I experienced it myself, but I did some work interviewing people at DV and GC. A person feels invincible and happy. Then they flop over.
If he collapsed from heat, yes, if his body was still operational, he would have headed for shade. But the increased body. temperature might have already been too high/ongoing.
Heat adaptation isn't a real thing. At least, it's not measurable nor is it absolute.
If the ambient temperature of the planet were to become 110, we'd all need A/C or our body temp would slowly go up. Just how slowly depends on many things (fat actually helps slow the process down a little, as the heat inside the body is dissipated into tissue that's unneeded for immediate preservation of life).
I've written a couple of articles on heat in the National Parks. 105F temperatures outside are fine for about an hour, but if a person simply stands, not exerting, in a 105F spot for a day, their internal temperature is likely to go up to at least 103F (which is not good - but survivable).
If it's 109F, DV puts up signs warning people not to be outside for more than 30 minutes (less if you're walking instead of standing). At 120F, they say no time outside the car (they used to say 5 minutes but I think it's 5 minutes at 115F now).
If a person is running, the times and risks have to be adjusted. People with larger body mass also have more skin, so can dissipate heat a bit that way. PK was not a big person and had very little fat, so those two mechanisms would not be available to him as part of an adaptation. I wonder if he had a white hat with a brim (the head really needs to be cool down). Water bottle and terry cloth lining to hat or similar really helps (evaporative cooling).