In addition to the early and fantastic planned mileage runs described above, I remember hearing in 1985 about an early, "unplanned" (not to mention *unfortunate*) mileage run.
I believe that after the TWA Flight 847 hijacking in June of 1985, TWA awarded mileage to the passengers on the flight. Specifically, they awarded mileage for the aircraft's actual route shuttling repeatedly between Beirut and Algiers, in addition to the originally scheduled Athens-Rome route.
I hope no one here had the sad misfortune to earn those miles.