It is one of the most amazing trails and definitely worth the detour if you can do it.
You don't say where you need to get there and back from, but assume the Norte - you should easily be able to get a bus from either Colombres or Naves to Poncebos. You would walk to Cain de Valdeon and then walk back again. It is possible to get a taxi at Cain de Valdeon but it will take several hours to get back via taxi and most people just walk there and back - it is possible to stay in Cain de Valdeon if you wanted to. It's about 4 hours each way. However depending on how much time you have and how adventurous you are then you could deviate off the Cares path on the return journey on the path up to the Covadonga lakes and then Covadonga which is a magical place also. from Covadonga/Cangas de Onis you can easily get back to the Norte.
If coming from the Lebaniego/Olvidado then you would head down towards Valdeon from Puerto Pandetrave (my second favourite place in the Picos).
If you had time you could pick up the Lebaniego from the Norte as this Camino cut's through some of the most beautiful and wild mountains.
Of course it is possible to deviate to the Cares from one Camino and continue on another one if you wished to.
It can be a popular trail and is very narrow in places with vertiginous drops in places so always try and avoid it on a weekend/public holiday etc to get the most enjoyment from it.
Just in case you need some encouragement:
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