This one can be farmed if you want to. I would suggest loading mission 7 (Pont Du Fahs Airfield) and getting to the snipers nest marked on the map with a red star. Make a save, shoot the longshot target (620+ meters) and then reload and keep doing it until you have the required distance.
When mine popped I had a shot distance in my service record of 85001.63 m.
EDIT: See ScuzzyBunny's comment below for a possible optimisation.
EDIT: The actual distance needed is 84,390 meters, thanks Shanevolution.