I have always said that the idea of long term survival is outside most of our abilities to cope. That was what I said when the original discussion about a long term survival forum took place.
Now, one small flaw in your math Susan, there probably is 300 million people and 9 million acres of designated wilderness areas. However, if everyone was in a dire situation, the concept of private lands means little, so there will be more than 9 million acres to live on. The problem will be defending one's space from people that want your piece of the pie.
While I do beleive someone can survive for years and even decades in the right circumstances, it is not likely an entire population can survive very long when faced with enough adversity and potential hostility.