Hey Kent... I just thought of something. What about the "wall"? I guess you do get it in both events, hiking Whitney and marathons. But, when I hit the wall in a marathon (and again, it's been almost 11 years), it is an extremely mental experience. I tell people it's like feeling "Why am I out here? Why am I running?" and I felt this between 18 and 22 miles and only once in a marathon and once in my first training run of 22 miles.
I get a similar feeling on Whitney and other hikes, but it's earlier, just a few miles in, AND then it goes away for good. When hiking Whitney with Gary, Bulldog34, I told him that sometimes I feel it on a hike and he said he sometimes feels it more than once at the early stages of a hike. Once again, it goes away for him (If I remember correctly.) And, it's always mental, not physical.
Just wanted to add that and see if anybody else has similar experiences.

"Turtles, Frogs & other Environmental Sculpture"
www.quillansculpturegallery.comtwitter: @josephquillan
If less is more, imagine how much more, more is -Frasier