I would argue that a marathon runner (esp one who is running most of the race and on roads) certainly has more short twitch never fibers than a backpacker. The speed of a marathon is much more intense than a 26 mile backpack.
http://www.bodybuilding.com/fun/kelly15.htm
(IS this where you got your chicken analogy from ??!?!! It is a good one BTW)
That 75/25 ratio (long/short) is gonna be even more skewed in dedicated backpackers vs marathon runners.
Ultrarunners (and trail runners in general) probably have the musculature/never fibers more analogous to backpackers esp those backpacker who hike long days/fast.
Go long..go slow is how most ultrarunners go (an elite ultra runner averages 6 MPH/10 min miles for over 100 miles in something like the Leadville 100. Impressive..but slow compared to the more intense and quick pace of an elite marathon runner. Apples and pineapples.. Kinda like backpacking and running. )
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How this is relevant to the OP, I do not know [/QUOTE]
And that is why many marathon runners cave on multiday backpacks esp of the intense long hiking day/elevation gain types.
Any person who thinks (road in particular) marathon running and mutliday backpacking is the same is well, probably not correct.
Running in a a marathon, on a road, with aid stations for one day is a different activity and skill set.
..and that is how it relates.