Hi!
I found a very interesting and helpful website about trekking in Spain. It has maps, pictures, even GPS routes.
I hope you like.

Topwalks

Thanks very much for the link Valenciano, I've spent many hours in the past searching for exactly that info, without success.

Shame there is nothing in the Madrid Sierra though, there is some really great walking up there.

It's fine as far as it goes, Damián but it just covers a small part of Spain: nothing in Vizcaya, Cantabria, Asturias or Galicia. If I were 'trekking' in Spain, those regions would be at the top of my list. The Camino Francés is notable by its absence, as is Extremadura and the western parts of Andalucia (Huelva, west of the Donaña).