Here’s a nice in-depth article on getting ready for thru-hikes. An excerpt:

I have gone backpacking in the Grand Canyon with marathoners who, by the end of a 12 mile day, became complete wrecks. Their endurance was good for comparatively short duration and they were not used to gaining elevation. Part of the problem was mental, but the other was physical: The muscles needed for climbing hills are not the same as the muscles for running on flat ground. Moreover, their marathons would generally be over in four hours or less. Hiking for eight hours or more is an activity significantly different from running for four hours. To illustrate the muscle concept, consider the following: In 2003, I hiked 2650 miles over 105 days on the Pacific Crest Trail. Upon my return, I began running again and assumed that as I had been, during the last month of my hike, averaging around 30 miles and 5000 feet of elevation gain per day, I would have no problem with my rolling 6 mile running route. I made it a mile and a half before the muscles in my legs quit on me. I wasn’t tired, but my trail-leg muscles were simply not helping me on the pavement. This, of course, is only a simple example, but it is one I keep in mind.

The author points out that the majority of people who go out for months don’t train beforehand — they just expect the first couple of weeks to get them in shape for the rest.

As we all know the only way to train for hiking is to hike, and it’s fairly obvious that if you had the free time available to train for months of all-hikes, you’d be taking months of all-day hikes.

I’m in the camp of “all training is good,” because the opposite of training is couch-potatoing (or, ahem, blogging). If you hit the trail out of shape, your odds of a hike-ending injury happening in the first few days have got to be way, way higher.

The other thing to be said for getting in shape is that there’s no reason why the first few weeks on the trail should totally suck, which they will if your bod isn’t at least minimally prepared for the abuse.