We also dug out millions of tons of useful materials out of ground, purified them and concentrated them in cities and along roads etc. We cleared forests on mos

We also dug out millions of tons of useful materials out of ground, purified them and concentrated them in cities and along roads etc. We cleared forests on most of continents to let people do farming there. In Eastern Europe at least the biggest limit on farming and state-building in early middle ages was clearing the forest for farming. It was the biggest energy expanditure for people there, and it took hundreds of years to get to a state in which most of the land is available for agriculture. If the apocalypse was quickly over - we'd get to continue from that point without spending all that energy again. If it takes 50 years or more - most of Europe would be one huge forest again. Depending on the kind of apocalypse all you'd need to do might just be to scavange for usable solar panels/wind turbines/car batteries and starting agriculture in the places that are still available. Or you'd need to start from scratch, but then we're talking centuries.