These "trail centres" of which you all speak - one of these days I'll have to get myself to one, just so I can say that I've been.
Sometimes it seems like I'm the only person who never visits them.....
As far as "natural" stuff goes, very little of it is truly natural anyway - sheep tracks, old mine and quarry tracks, pack roads, drovers roads, all these places that I ride were made by someone (or sheep). Even the off-piste across the moorland stuff would have been vastly different before deforestation and now most forestry around here isn't any older than me - if I go riding round my local forest I'm the oldest thing there, as planting didn't start until 1956
In Greece it's the same, I'm riding on old Kalderimis (mule paths), newer agricultural dirt roads and goat paths (like sheep tracks, but steeper and rockier then).
Very little around here is truly natural and, ironically, what is usually isn't very rideable