The longest regular journeys by 1st Generation DMU in Scotland would haven been Aberdeen to Inverness, which is about 105 miles and took over two and a half hours, using class 120s. Glasgow to Stranraer is slightly less distance and was about the same time, using class 126 DMUs. There was one "fast" DMU service from Glasgow to Oban which ran via the WHL to Crianlarich prior to the Callander route closing (again about 105 miles), think that used class 101s and would have been a real marathon.
Jim