I agree with you insofar as he likely traveled quite the distance on foot. He could have unintentionally wound up back in that particular field after running/walking -- who knows how far or for how long -- while disoriented or hallucinating, in a panic, not knowing where he was, thinking he was being chased, or who knows what? Exhausting himself until he sat/laid down in that field and succumbed, I assume to hypothermia.
The conjecture I'm reading and the "trying to understand and explain" assume a rational state of mind. That David only walked 1/2 mile since that's how far his body was from the truck, that he would choose to walk southbound on the roadway, that he would have recognized and gone to a farmhouse if he needed help, that someone other than himself put him in the field, etc. So far, the evidence isn't telling us any of those things.
We are trying to find a rational explanation for behavior that was clearly irrational. I get it, it's human nature to superimpose our own states of mind on others, and on David in this case.
How likely is it that details about what led up to his death won't be possible given the months that passed and the state of his body upon discovery. And by that, I mean blood samples or other samples that would provide a definitive toxicologic explanation?