Information about Ruta del Cares can be found here:

http://www.jcyl.es/jcyl/cict/dgt/svfit/turismo/descubre/ecologico/espacios/piceur.htm

and here:

http://www.cabrales.org/2/frutas.htm

Photos of Naranjo de Bulnes and others can be found here:

http://weber.ucsd.edu/~rdesanti/Picos/picos0.html

Looks quite rugged to me. I know I won't get an answer to this anytime soon but I wonder if it is a six mile trip one-way or round trip?