The Difficult is that which can be done immediately; the Impossible is that which takes a little longer.

George Santayana (1863-1952) Spanish-American poet and philosopher [Jorge Agustín Nicolás Ruíz de Santayana y Borrás]

(Attributed)

Quoted in Reader's Digest (Nov 1939), but without citation. The sentiment has a number of antecedents (see discussion here).