Bootstrapping the reeducation of enough people to effectively get them intensive-gardening in that kind of a situation would be a massive challenge, but if enough people did the feed-your-neighbors preparedness approach we might have enough time to get it done. If we were looking at a long-term scenario I think I'd want to have at least a year of grace period to get people rolling. Thinking back to all the mistakes I made starting my own farm, if my life had been depending on success I'd most likely be dead, or would have survived solely by eating my sheep...

Cuba is an interesting example of a place that had to rapidly adopt a low-power lifestyle, and they actually seem to have done it with grace. There might be a good case there to study for clues about what works. They have the advantage of not having to contend with long, cold winters though, which is a critical difference for most of North America.