According to Wikipedia, the runway at the Ulyanovsk Vostochny Airport is tied for longest in the world (at 5000 meters/16,000 feet), in addition to being the widest (at 105 meters/344 feet).
It's not clear to me why this is the case: usually a very long runway is required at high altitudes or in hot climates, and neither is the case for Ulyanovsk. The Shuttle Landing Facility is also particularly long and wide but it was built for a special application (and it doesn't even make it in the top 10 in that list). Was the runway at Ulyanovsk also built for a special purpose? It can't be for the sake of the An-124s manufactured nearby, as those (obviously) can operate from conventional runways.