The more I read about the deviation from his normal routine and habits, the more I lean toward a voluntary disappearance. The only thing I can't make fit is fact that he didn't have that much time before he was supposedly needed to pick up the family. Unless that plan was added last minute and he couldn't really argue about it, it wouldn't make sense not to give yourself more time.
As a runner, I have to say, it is important to pay attention to normal routines and habits. Also, I wouldn't run more than 5 miles in the summer heat without at least a small hand-held water bottle. When I was a novice and tried this it never turned out good and was a waste of a long run. Maybe some can pull it off but I don't see it being likely.
Also, there has to be someone on the trail that saw him. As a woman who mostly runs alone, I can say I notice everyone. On a weekend day there had to be many people out at the trail walking dogs or even some very dedicated individuals biking or running in the heat as well. Who has come forward to say they saw him that day?