I think there is another issue with a 400 mile foot journey as a backup plan beyond the logistics of the trip itself (which, alone make it a non-starter for me). The bigger issue for me is that I can't envision a disaster that would make such a journey necessary.

For example, if my entire city were effectively laid waste and made inhabitable by some sort of natural disaster my closest family would be close to 400 miles away ... it is really the only place I could set down for an extended period of time in such a situation. But that doesn't mean I need a plan in place to walk there. Even in the unlikely event that I couldn't get out of my city with a vehicle, but somehow did get out, there would be help available closer than that - at least short term help (churches, Red Cross, etc).

Even barring finding short-term relief (maybe all the support organizations are all overloaded?) I think I'd be looking for some form of alternate transportation, like bus maybe, or even just a place to hole up so I could arrange for help from those outside of the affected area.

In short, any disaster I can envision is going to be localized and there will be safe places to go closer than my family 400 miles away. And if there isn't, what guarantee would I have that my family hasn't also been impacted and displaced?