To get the Compostela certificate issued by the council of canons of Santiago de Compostela Cathedral, a pilgrim must complete at least 100 km of the Camino on foot, or 200 km of the Camino by bicycle or on horseback. Anyone who begins their Camino here—which is right on the 100 km mark—would already be entitled to qualify for the Compostela certificate upon their arrival to Santiago de Compostela.
No public transportation is available between Sarria and A Pena. Sarria is the most common starting point for those walking the last 100 km as public transportation only gets there from Santiago de Compostela.
From Santiago de Compostela (or Santiago de Compostela Airport), there’s a bus going to Lugo.(See here). DIn Lugo, you’ll have to change buses as another company (See here), does the route to Sarria. Afterwards, many people start walking from Sarria, which is approximately 114 km from Santiago de Compostela.
Another way to get to Sarria is by traveling with Renfe, as there are regular trains from all over Spain, particularly from Madrid and from Barcelona.