Used to be penances given were so big, and lasted such a length of time, people would ask for substitute penances that were harder, much harder, but shorter in time. I.e. equivalent in punishment, making up for the less time with more pain.
So you could substitute several years penance for weeks or months doing some very tough activity.
Or you could go the other way and lengthen a difficult one to be easier in parts.
Of course to be allowed this is a mercy that was not always given.
Back in Ireland way back, for example, a substitute penance, or commutation for a black fast for a great crime could be one hundred blows with a scourge.
Here's from an Irish Penitential book:
'An arreum for a black fast on account of a great crime for any one who does not read, viz. 300 genuflexions and 300 honest blows with a scourge; crossvigil at the end of every hundred until the arms are tired; “Oh God I beseech an end, may mercy come to me,” “I believe the Trinity,” this is what he shall sing without ceasing until the arreum shall have come to an end, and strike his breast frequently at it with earnest repentance to God.'