The US has quite a bit of arable land still available. Estimates on the Web range from 302 million to 470 million acres. Since I am a sustainable farmer, I know how much food I can grow, how many potential calories are available through human labor, and other parameters for how many people we can actually feed with only minimal use of petroleum products. Let's take the low figure of 302 million acres of arable land. Certainly this includes land which must be irrigated, but there were several traditional cultures that managed to grow food in extremely arid environments and did quite well. One example was the Hohokam in the area now known as Phoenix, Arizona. They had an extensive network of irrigation canals that could conceivably serve as models. There are ways to do irrigation that are relatively low impact and many farmers and environmentalists are advancing this approach. In other words, irrigation should not be a limiting factor in how much arable land can be used for production.
What about labor then? What intensive petroleum use does is allow many more acres to be farmed by fewer people. Yet we have a lot of people that will soon be out of work, if they are not already. One of the "untouchable" subjects in most discussions of transitioning to a saner society is the idea that we will have to dramatically increase the number of people doing manual labor. My estimate is that we will need 10-20% of the population actively working full-time as farmers. So . . . as a farmer, I will have to feed myself and either 4 other people, or feed myself and 9 other people. This is certainly doable as I now produce enough calories on 2 acres to feed approximately 10 people with a daily calorie requirement of 2500 calories.
Is land use and ownership a problem? Yes it is. However, since we are talking inevitability of dieoff unless we act like an aggregate of local communities, I don't see land use as continuing to be as narrowly strait-jacketed as it currently is. In other words, people with land will be forced to realize the benefits of growing food and will have people ready to work the land in labor-intensive, small-scale agriculture. A land of small farmers growing food on postage stamp-size lots is not just a fantasy, but rather the likeliest way that the future will develop, given the lack of leadership from government, coupled with the intransigence of public employees (elected or not) to do anything without extreme prodding.
To sum up, how much food can we grow on 300 million acres of arable land? I suggest we can feed 1.5 billion (300 million acres X 5 people per acre). Many of you old fogey types (and even some of you "new" fogey types) will scoff, but I suggest we can actually feed the world via small-scale intensive agriculture. When oil was cheap, millions were starving because of inadequate world-wide distribution and corporate greed. I suggest that the future starving millions will still be the result of inadequate world-wide distribution and corporate greed, NOT because we cannot raise the food without cheap oil.