Not the longest distance, but the longest overall ache: Hiller 12E, gaggle of four, from Culdrose to Nuremberg (sp?) and return. Took a day each way to get from Culdrose to Manston
And of course, rubber boats had to be worn for the Channel crossing, which took up 90% of all available baggage space for the rest of the trip
4 days each way, IIRC, and numerous stops to recover. Memorable was the flight of F-104G's that cruised by under us, fortunately we were at a nose bleeding 500 feet..........