Advocates of sustainability often sound like they are pushing for either of two extremes. You can get a plot of land way off in the hills and do everything yourself, or you can move into a dense urban center so you can walk to everything. Each has its own issues based primarily due to reality.
There's just not enough arable land on the planet for 7 billion people to carve out individual plots of land. Also, you're not going to get everyone in the developed world to give up modern life.
As for moving to the cities, there are many hurdles in getting people to move to dense centers, but I think most of these could be overcome. Many are afraid of higher crime. Many people (like me) who care about sustainability also care about the environment. It's hard for us to leave it all behind. This is the real issue with moving everyone to the city. How do you get subsequence generations, raised in the urban jungles, too care about these issues? Ironically, it doesn't seem sustainable socially.
My solution is to focus on many, small dense centers. Economics will continue to support the large urban hubs, but we need to shift satellite developments from sprawling subdivisions to dense, centralized communities surrounded by a combination of conservation and farm land. These centers could be linked by mass transit to the big hubs. They'd provide all the green benefits of living in a city while still allowing residents easy access to open areas.
I'm sure this is not an original idea, but it never occurred to me in California. Having been back in Massachusetts for a while, where the country side is dotted with town centers that developed before the automobile, it's clear to me now. I'm not sure how we get there, but I think it's possible. It should almost be easy in this area.