Here are some videos which will give you an idea of the terrain.
This is the Cares Gorge, as described in Walk 26 of my Picos book, although the video goes in the opposite direction. https://www.youtube.com/watch?v=2bi6HFxX2-s
And here is a mid-winter video of the route to Pico Tesorero. You will see how difficult winter conditions can be in northern Spain. https://www.youtube.com/watch?v=Z7TsmRU7fKg. The first section of this route is the same as in walk 17 of the book.
I will shortly add some videos from the areas covered by my other books.