Totally agree. He wasn't a big guy. She was tiny though. But hey that's what tarps are for...to drag things on if too heavy. That and other things.
I am picturing the regular, plastic tarp that people use under their tents (I think we can see it in Gabby's tent video). Not the old school canvas, padded tarps that were in Army surplus stores after WW2.
If that's all Brian had in his backpack when he got into that couple's car, that's very strange.
I'm not seeing how people go "camping with just a tarp" in that region (cold at night, other reasons - such as predators). I guess one could fold it and use it like a bivvy (roll up in it). I've never seen anyone do that. 2 tarps (one strung over a string between trees and another to lie on), I've seen as part of beach camping.
There are enough trees to sling it up (if he had cord) on the route between Spread Creek and Colter Bay. Then he'd have to just lie on the ground under it. Presumably the van was back at Spread Creek.
Why would he do this?
One theory is that Gabby is dead (possibly in the van?) and he can't handle driving the van or disposing of her body at that point, so starts off to Colter Bay, sleeping some along the way. Gets a shower there, but what was he thinking he would do then? Obviously, he eventually goes back to where ever the van was.
I think it had to take him at least 5 hours to walk.
In another scenario, he didn't walk from Spread Creek at all - he hid her and took the van to Colter Bay and then
pretended he needed a ride. (someone else suggested this upthread) so that he could create an alibi. Maybe he contacted/sought rides with several people, hoping that some of them would remember him.
Not sure how that would have helped him much, but there's something off with his thinking at that point, perhaps for quite a while.