I had a look, out of simple curiosity to see where San Sebastian de Garabandal, which I'd never heard of, is.
The place looks stunning but all the routes suggested involve long taxi journeys to the village.
Having found myself having to hitch-hike earlier this year despite R2R's information that a journey from Plasencia to El Torno could be done by bus and taxi, I'd suggest that you try to contact someone in the village to check the taxi situation in advance, and book one.
Or maybe a tour from Santander would suit you eg
https://touristjourney.com/tours/mo...de-garabandal-pilgrimage-tour-from-santander/
No problem at all travelling around Spain as a solo female. Just the normal precautions about money/passports and care in crowded places. The only thing I still find awkward is eating alone in the evening., so I usually eat in the afternoon, like the Spanish do. But that's me, and just a couple of encounters with waiters unused to solo female diners.