The Appalachian Trail is somewhat in excess of 2,000 miles and a LOT of people have completed that hike. So, rather than making a monster out of a 400 mile trek, look at it as being just 20% of the AT. At ten miles per day, it takes 218 days to hike the AT. Allowing for 30 "zero" or "nearo" days, that's a shade over 8 months. That's doable.
Applying the same math to a 400 mile trek, we get 40 days of active hiking. Allowing one day of rest per two days of hiking we get a total of 60 days, or two full months. That's
very doable.
If you break a 400 mile journey down into increments of roughly 10 miles per day, there would be plenty of daylight left to set
automatic fishing reels* and / or
fish traps* in any promising body of water nearby while
setting snares* or
foraging for wild plant foods* . Unless you are being pursued, if the food supply where you are is plentiful, stay there a couple days to recharge. Smoke or dry some of the fish or game, dry some of the fruit and roots, eat as much as you can, then stand up and resume the journey.
I'm not saying that a 400 mile hike to find help is desirable or even reasonable (was there no kind person to be found any closer?), only that it is doable.
The best way to train for something is to do it. You don't train to run by doing push-ups and you don't train to lift weights by taking long walks.
My information about hiking says that, within a week (in most cases), the body responds to the new demands being made on it and that 1,000 pound pack will seem to be a whole lot lighter and there will be a spring in your step that hasn't been there for years.
I'm not making light of that long journey, only trying to be reasonable in my appraisal of it. It
can be done. It
has been done. It
is being done even as I write this (Oct. 23, 2012) and it
will be done -- again and again.
The most serious hindrance that might keep any of us from making such a journey, should it become our best option, is fear. Conquer that and the rest will fall into place.
* Legal Notice: such activities may be regulated or even illegal in some jurisdictions. It is your responsibility to know and obey governing law. If other humans are nearby, you might find the necessary nutrition at lower caloric cost to you by either buying food or dumpster diving. If you've been out on the road for a while, you probably look and smell like a bum and would thus fit right in with the other folks checking the contents of bins. With survival at stake, how many inhibitions can you afford?