Musar do Diwrej ha-jamim I 29:15

For seven reasons should a man repent early while still in his youth when his power is strong. The first is that the labor needed to acquire Torah and reverence for God and all qualities that a man must possess is exceedingly great. And concerning this it is said, "The measure thereof is longer than the earth and broader than the sea" (Job 11:9). And, "The day is short" (Aboth 2:20). For this world is a very short day. "Like a shadow are our days upon the earth" (I Chron. 29:15). And our Sages, of blessed memory, said, "Not like the shadow of a wall and not like the shadow of a tree, but like the shadow of a bird in flight" (Gen. Rabbah 96:2). And as to the expression, "And the workers are lazy" (Aboth 2:20), this refers to a man who has within him the quality of laziness.