Spain (like space) is big. Not big on a Texan scale but a lot bigger than Portugal and nearly as empty. You can walk for a day in Extremedura, cover 35 Km and see some of the most spectacular scenery - without ever seeing another person. But it's not a wilderness; there are signs of human habitation - and no doubt if you needed to find some people it wouldn't take long. But if you don't then there's good chance you won't. Try the walk from Casas de Don Pedro to Valdecaballeros through the hills; its highlighted in red.