Cultivation was different from martial arts practice; after martial artists improved their cultivation level, their combat power would leap qualitatively.
Yet Cultivation of Truth was different; every major breakthrough would extend a cultivator's lifespan. Under normal circumstances, a cultivator who practiced ordinary cultivation techniques would, after reaching the Foundation Establishment Stage, have a lifespan of at most around three hundred years.
However, the Longevity Skill you two have been practicing happens to fall under those abnormal cases. The Longevity Skill I found for you two might not be very strong in actual combat, but with this cultivation technique, every breakthrough grants you much more longevity compared to other cultivators.