But I never was happy, never could make a good impromptu speech without several hours to prepare it.

Mark Twain (1835-1910) American writer [pseud. of Samuel Clemens]

Speech, Society of the Army of the Tennessee Annual Meeting, Chicago (12 Nov 1879)
    (Source)

Twain made quips along this line several times, though there is no evidence he said the more frequently quoted "It usually takes more than three weeks to prepare a good impromptu speech." For more discussion, see here.