Set your stages by your own ability, the terrain, the spacing of places to sleep, and above all the number of days in a row. A distance that feels easy once at home feels different on the fourth consecutive day. American Pilgrims notes that many people begin with shorter days to let the body adjust, and ease off later with a short day or a rest day when they need one. Most walkers reach their stopping place between 1:00 and 3:00 p.m., which leaves the afternoon for washing clothes, rest, and the evening Mass.
Shorter days do not put the Compostela at risk. The Pilgrim's Reception Office asks for at least the last 100 km on foot, with two stamps a day in the credential over that distance, however many days it takes. What does matter is walking all of it: skipping a stretch by bus or taxi within the required distance can cost the certificate. So plan each stage to end in a village with a bed, and collect the two stamps as you go.
Good to know before you set out: if you are joining an organized Camino, ask for each day's actual distance and climb and what happens if you cannot finish a stage, because an average can hide the hardest day. A luggage transfer service, such as the one Correos runs for bags of up to 15 kg, lets you walk with only a daypack. And train for consecutive days, not for one long walk.