The best you could hope for is an internal split into towns and villages with live agricultural centres surrounded by dead zones.

Bulldozing houses is much quicker and cheaper than putting them up.

But anyone with a hankering for pastoralism needs to consider that a new dark ages will inevitable create a new war lord class, intent on reducing most of the population to feudal slavery just because they want to, and can.

The most likely outcome is a pre-medieval stockade system, with heavily fortified population centres in the middle of agricultural land, where the peasants are largely considered expendable.

You may be able to tend your homestead in peace in less densely populated areas, but it's not going to be an option within a couple of hundred miles of any reasonably sized city.

NB: My own dream has been to haul off to a ruin in the French countryside and build a self-sustainable homestead.

But, but, ahem, what John Michael Greer has to say is sobering.

Equally imaginary is the notion that the best strategy for would-be survivors is to hole up in some isolated rural area with enough firepower to stock a Panzer division, and wait things out. I can think of no better proof that people nowadays pay no attention to history. One of the more common phenomena of collapse is the breakdown of public order in rural areas, and the rise of a brigand culture preying on rural communities and travelers. Isolated survivalist enclaves with stockpiles of food and ammunition would be a tempting prize and could count on being targeted.

So what does work? The key to making sense of constructive action in a situation of impending industrial collapse is to look at the community, rather than the individual or society as a whole, as the basic unit. We know from history that local communities can continue to flourish while empires fall around them. There are, however, three things a community needs to do that, and all three of them are in short supply these days.

The pirate enclaves of the seventeenth-century Carribbean were among the most lawless societies in history, but physicians, navigators, shipwrights, and other skilled craftsmen were safe from the pervasive violence, since it was in everyone's best interests to keep them alive.

The second thing a community needs in the twilight of industrial society is a core of people who know how to do without fossil fuel inputs. An astonishing number of people, especially in the educated middle class, have no practical skills whatsoever when it comes to growing and preparing food, making clothing, and providing other basic necessities.

Well I have a number of practical skills, and I can learn more!, but in isolation they won't do me much good. Greer's analysis is well taken.

That's about the size of the city where I live.  I reckon you can more or less see the city limits from a (not so) tall building or vantage point.

Were DeAnander and DoDo talking about this size of population?

Put another way, maybe you can have all the top class science (well, not all, but a lot) going on in a town of 300-500,000.  They would be city states, perhaps, so following the less dystopian line it may be a return to something like Italy after the plague (as you said a while back), only with better connections (no reason for entire networks to collapse I don't think), and 21st Century know-how in agricultural practices, building, materials etc.

Or, to put it another way, England has an area of 130,000 sq km and a population of 61 million--London has a population of 7 million (and the rest!); Bulgaria has an area of 110,00 sq km and a population of 7.3 million; this site gives me 43 cities with populations over 7 million, (468 with populations over a million)...

So...I'm suggesting that what was once a city would today be called a medium-sized town.

Another way of thinking about it ("it" being, how could you survive without a modern civilisation around you) is: how reduced does the world's population have to get before you see the disappearance of centres of learning?  They existed in 500BC (world population: 100 million--that's the combined populations of Tokyo, Seoul, Ciudad de Mexico, and New York (more or less.)

So, if we see a collapse of world population...hmmm..

6.6 billion today?

-90% = 660 million?

-99%...

6,600,000,000 / 100 = 66,000,000?

So around 500 AD the world population (300 million) was equivalent to 5% of today's population?

Of course if life dies back to small mammal level, it might be trickier...

But I'm assuming that survivors of any catastrophes will be:  (add AND/OR between each item)

Intelligent
Farmers
Rearers/butchers of livestock
Materials specialists
Builders
Educators
Computer types
Sailors
Mechanics
Bakers

etc...

Plenty to build a complex civilisation...in fact, I doubt there has ever been a society that completely lacked an aspect of its existence complicated enough for you to find enjoyable....(he types wildly.)

Don't fight forces, use them R. Buckminster Fuller.