That food production must become drastically more local is not in doubt. The manner of that production will have to change and whatever comes next probably won't be able to keep the number of humans in the billions. Industrial agriculture will no longer be possible, but even organic agriculture may not likely play the role in our future that it has in our past. Intense local horticulture in the form of ubiquitous urban gardens may seem the Pollyanna whimsy of Utopians, but vast urban horticulture may be one of the rafts that will collectively keep civilization afloat. Despite the optimistic projections of proponents of this idea, however, I doubt that this sort of permaculture will be enough to support current population levels much less more growth. It will entirely replace industrial agriculture and supplement organic agriculture, but it will not increase the planet's caloric carrying capacity enough to allow further growth or save billions of lives. Part of the deal is healing our degraded environment and you can't guide energy back into the soil and have enough energy left over to feed people. You don't get to hold your energy cake after you've eaten it. We could probably establish a permanent horticulture, but only while letting the number of humans diminish. There is a cost to everything. Some of the population is going to have to go back into the soil. This ups fertility while reducing the number of mouths to feed. It's an indirect form of cannibalism, but it's mild compared to the more direct version to which starving people will resort.
The most natural and least ecologically harmful method of procuring the calories to sustain human life is foraging in smallish nomadic tribes. Conversely industrial farming to sustain increasing millions of metroplex inhabitants is about as bad as it can get. Traveling foraging populations are directly limited in size by the availability of nutrients in the form of naturally occurring flora and fauna. If an area is always left to recover after a relatively brief period of fairly gentle exploitation by reasonable, small numbers of humans, then indefinite sustainability has been achieved. It's what all the other animals do. They scrounge around to find edible leaves, nuts, berries, bugs and other prey animals. They take it as they can get it. Shortages of one food source quickly lead to declines in the numbers of the things that rely on that food source and the food source itself then gets a chance to recover...normally. Negative feedbacks can be established and extinctions can occur (in fact, they occur with mind-boggling frequency in geological timeframes). With the exception of humans animals don't have deliberate agricultural practices. Other ecological checks and balances keep things like relative population of predator and prey stable overall unless some unforeseen outside event introduces a new factor. A limited number of humans who avoid setting up house in any one area for too long practice a form of light exploitation rotation. Staying in place and cultivating a food source and storing it are deliberate and uniquely human activities that can throw things out of whack at varying speeds. As soon as a band of humans sticks to a place and has a more reliable (if ultimately unsustainable) means of procuring food and storing it, then human population will increase geometrically as much as the growing surplus will allow. With the industrial mode of agriculture the land never gets a chance to recover from the relentless mining for nutrients. Overshoot, crash and dispersal of the remnant are imminent. I've read convincing arguments that any form of agriculture is ultimately unsustainable, but I've also read that very careful applications of traditional agriculture may reverse desertification. The best I can surmise is that with enough care, agricultural practices resemble horticultural ones. There is a continuum displaying an inverse relationship between efficiency and intensity from hoticulture to industrial agriculture. The costs of increasing absolute caloric yield is an accelerating decrease in efficiency and all the problems inherent in expanding populations surpassing (relatively rapidly) what can be sustained indefinitely.
Simple horticulture, though it too is a means of diverting more nutrients into making more humans, isn't the same sort of strip mining as agriculture nearer the industrial end of the scale always proves to be and can supplement the food cache for semi-nomadic foragers. Several horticultural sites can be established and their use rotated as the tribe makes its rounds. It's really just stewarding a miniature ecosystem and foraging for edibles in it. There is some threshold where stewardship and coaxing food energy from the ground becomes ecological rape and ruin, from personal garden to agribusiness. A garden is like a small, personal jungle full of nutritious plants. Agriculture introduces the use of monocultures grown in segregation on larger tracts of land. At its most intense agriculture essentially stamps out biodiversity for a more streamlined means of producing food for humans and their work animals. Doing such sets up an imbalance whereby the ability of the soil to sustain life is slowly eroded. Meanwhile, agriculture also allows for surplus which helps short circuit the natural feedbacks that keep population size in check. So even as agriculture forms the base for population increase and the societal complexities and role specifications collectively known as civilization, it is also a recipe for slow motion overshoot as it transforms soil into increasingly urbanized humans. I believe agriculture and civilization can be good neighbors that exist in balance with the rest of nature if humans consciously limit their numbers. (Here I will take the opportunity to point out that while so-called market forces are oftened accused of not being able to act quickly enough to affect individual choices to limit family size, some of the worst examples of rampant, imprudent breeding I have personally witnessed have been by those with the fewest resources as subsidized by the state and encouraged by state policy; the state itself and growing centralized control corelates to the absolute yield of intensifying agricultural practices--a positive feedback loop that ensures surplus will lead to overcomplexity and overshoot. The state is the summation of the biological urge to exploit resources and reproduce and thus tends to favor ever-expanding population even in the face of ecological disaster.)
What I can't imagine is how quickly urban and suburban horticulture will step in to compensate for the demise of industrial agriculture and the 3000-mile food supply lines. How many suburban lawns and tenement rooftops will actually become gardens and how many farms will be reestablished in the parking lots of the exurban mega-shopping centers? I believe the transition of the metroplex back to the small cities, towns, villages and farmland it smothered may prove to be impossible. What agriculture remains will be organic and will probably more closely resemble horticulture. The catch, as mentioned above, is that we have already used all our readily available hydrocarbons to rip most of the useful stuff out of the ecosystem and turn it into human bodies. We are left with a degraded environment and what fraction of the current world population can truly be sustained with a combination of agriculture, horticulture and hunting is any body's guess. There are 600 million tons of fertilizer locked up in the form of bipeds with visciously destructive habits. Some large percentage of this biomass may have to go back into the ground to fertilize it and the population will in the future ever be under the pre-hydrocarbon ceiling.