Well, I wish I had gotten to this story sooner. There was so much that needed to be said about the science here!
1) As several have already mentioned, an impact that annihilated all of North America would have resulted in a global catastrophe.
a) Large impacts wreak havoc on the atmosphere, creating a hole at the impact site and turbulence globally
b) Material ejected from the impact site travels ballistically and heats up the atmosphere on re-entry causing global wildfires
c) Dust created at the impact site is lofted into the stratosphere and distributed globally in a matter of days, reducing the total amount of sunlight reaching the surface and causing a "global winter".
So, the actual kill mechanism is not getting hit by the impactor or even the materials it ejects to the immediate vicinity, but rather all these global effects, which make your distance from the impact site irrelevant.
2) A globally catastrophic impact can be caused by a meteorite about 15 km (~10 miles) in diameter. The resulting crater would be about 200 km (~125 miles) in diameter and the area affected by thick deposits of ejected material would extend to about 1000 km (600 miles) in all directions from the impact point. Everything beyond that would get pretty much the same effects as anywhere else on the globe (see point 1 above). For the catastrophe to be local instead of global, these numbers would all be correspondingly smaller - so we can consider these a maximum. For reference, the distance between New York and LA is about 4500 km, so there would be an awful lot of North America that was no worse off than Europe, Africa, or even Australia. If an evacuation is organized properly, people can easily drive out of the danger zone.
3) Once the global winter is resolved, there is no reason why the impact site and it's immediate surroundings would be unsuitable for farming (it's not a nuclear explosion with radioactive fall-out, after all). Also, impact ejecta deposits should make excellent soils. They resemble volcanic ash or glacial tills, which are both very good growing substrates. Organic fertilizer would be required to speed up recovery, but all animals produce that on a daily basis.

4) A 200 km impact crater is not like a sudden hole in the ground. There's a rise towards the crater rim, then a descending slope inside, then probably another rise towards some kind of central structure, probably a peak ring. The slopes are mostly on the order of 20 degrees or less, so certainly walkable with care. Of course, that's an average and there will be the occasional very steep section, but if you can avoid those on the way in, you can avoid those on the way out, too.
5) There is nothing "unnatural" about an asteroid impact. It is a very natural process that has been going on in our Solar System for at least 4.2 billion years.
Now that I have that out of the way, I can honestly say I liked this story. It was very interesting to see the human aspect of a recovery from global disaster, even though the details of that disaster were way off.