many years of stress and anxiety (as would cause one's hair to grow white)
Literary expression, often used hyperbolically to describe deep, long-lasting worry or hardship. The literal meaning is 'three-thousand jō of white hair' (about nine kilometers), a poetic exaggeration.
See also: 丈 (じょう)
It is no exaggeration to say his hardships amount to 'three-thousand jō of white hair'.