Happiness does not come from doing easy work but from the afterglow of satisfaction that comes after the achievement of a difficult task that demanded our best.

Wilferd Peterson
Wilferd A. Peterson (1900-1995) American writer, magazine editor
“The Art of Happiness,” This Week Magazine (1961-02-04)
    (Source)

Collected in The Art of Living (1961).

Almost universally credited, without citation, to Theodore Isaac Rubin, but I've been unable to find the phrase in Rubin's works or credited to him earlier than Peterson's essay.