In reply to Bluebird:
We did 4 days walking in May/June 2014
We flew to Santander, & stayed in Tielve, which is a little village in the North of the Picos national park:
http://www.toprural.com/Casa-rural-alquiler-%C3%ADntegro/El-Collad%C3%ADn_2...
The nearest big town was Arenas de Cabrales, which has supermarkets & stuff. If you were based in or near Arenas, it wouldn't take long to drive into the park.
We had two guidebooks, but they didn't help very much.
The park has a website with the walking routes:
http://www.magrama.gob.es/es/red-parques-nacionales/nuestros-parques/picos-...
There's a board with the routemap, & distance/time at the start of most of the walks. The times are for a fit party with no stops to gawp at the amazing scenery, take photos every 100m, bath your feet in the amazingly blue water of the River Cares, etc (I'm sure you get the idea - realistically add 25%)
The 'official' paths are pretty much the only viable routes without climbing gear. They take sometimes improbable lines through bands of limestone cliffs. They're reasonably well signposted, but you will need the 1:25k Adrados map.
Ruta De Cares was the highlight of the trip. If you do it, take care to locate the path that heads up & off to the right almost before you've left the carpark, don't follow the one down by the river (it comes to a dead end after about 2 miles... as we found out).
You can walk up to the Urrieles hut & back down in a day if you start from the Col between Sotres & Bulnes (drive up the off-road track from Sotres till you run out of track & just can't go no further). Or you could do an overnight in the Urrieles hut starting from Poncebos bridge (via Bulnes).
Other people we spoke to had done the Lagos de Covadonga walk, & recommended it.
We only visited a small corner, & I'd definitely go back, so I'm sure you'll enjoy yourself!