Kiashu - thanks for info and links, but I have some reservations about your viewpoints. I think you'll agree that we need to be wary of too much generalisations, as what applies in one locality does not apply in another. Before this page gets too passé, I'll just comment on the locality I know well which is the UK.

You say lots of fruit and veg produced in market gardens near cities. Supposedly 200km is near in your calculations. If you had to cart that food without oil you'd not want to be carting it more than 10-20 miles max.

As a cyclist I have intimate familiarity with the countryside around Birmingham UK. There's precious little growing of fruit and veg anywhere within 20 miles. Most farms have closed down since the commuter-belt land/property is now too 'valuable' for mere food production. In UK/Europe we can't grow most types of fruit anyway and so most of it is imported from distant continents, even apples.

You appear to consider meat to be an unnecessary abuse of grains. But many people in Northern lands need a high-meat diet to be healthy. I myself have to avoid grains with exception of rice. And livestock can graze on land that cannot otherwise grow food.

Yes there's a high productivity of food in UK/Europe but it is high productivity of unhealthy junk at serious cost to the environment. BSE, foot-and-mouth, TB and blue-tongue plus the constant colds of cattle reflect this unhealthy trashiness.

The Farmer's Guardian newspaper presents a highly mechanised, high-energy system preoccupied with trying desperately to keep heads above the profit/loss waterline. A prime organic farm explain how they take their sheep on an hour's journey to the abbatoir, then back again. How's that for energy resilience? All the small local abbatoirs have closed down and aren't going to reopen anytime imaginable.

Now I shall give more answer to your maths question. Medieval Britain (~UK) was struggling to feed its much smaller population, even though its land was a lot less degraded back then. The whole reason why the industrial revolution started here (nb) was that land-resources were becoming scarce. In particular in this naturally wooded land there was a shortage of wood. Ladywood where I live had ceased to contain any wood 500 years ago, having been burnt as fuel. Coalmining was developed as a reaction to the shortage of firewood. (See also Karl Marx's crucial life-event re shortage of firewood.) The coalmining had to go deeper and so the steam engine got invented and the rest is increasingly-polluted history.

The ChrisMartenson crash course illustrates how the growth of population stayed low for zillenia till there was the sudden growth of the energy supply. From the population history one can reasonably reckon the UK could at best manage 1/3 of present population. But we are very far from an at best situation. We have degraded land; we have farms that are no longer farms; we have huge urban populations living far too far from the fields that will need to be worked. We don't have the cash to do that relocation even if the rural areas weren't inhabited by prickly rich people upholding their property rights and strict anti-development regulations! In short we're rude word beginning with f-'d.