A pilgrim who cannot finish a stage can stop at any village with a bed, or take a taxi to the planned overnight town and resume walking from the same point the next morning. Skipped kilometers in the final 100 km are not credited toward the Compostela, so the missed section must be walked, in order, before continuing.
Older pilgrims commonly plan rest days and shorter stages: splitting Sarria to Santiago into seven or eight days keeps most stages under 16 km, and a rest day in Portomarín or Arzúa breaks the walk in half.