Weren't endurances of 20+ hours doable in the latter part of the prop age? I remember reading somewhere that TWA's Super Connies operated U.S. West Coast to Europe nonstop (eastbound only). That would be a flight time in the neighborhood of 20-24 hours, I imagine.
Come to think of it, those Pan Am Clippers across the Pacific must have also had some mighty long sectors (even with stops at places like Midway or Wake Islands).