This day is mostly about perseverance. You will be on the road for around ten hours, with plenty of short climbs and descents but no real mountain. It is long rather than steep. Eat enough along the way and walk at your own pace.
At Boo de Piélagos you have to cross the ría de Mogro. The official route takes a detour inland via Puente Arce. Many pilgrims take the train at Boo instead, one stop to Mogro. Never walk across the railway bridge: it is forbidden and dangerous. After that, you carry on to Santillana del Mar. The day ends there on cobblestones, among old stone houses that seem to have stopped time.