According to an article published on CNN,
Referees, who can be twice the age of the players, sprint across the field keeping up with the players and closely tracking the action. They tend to run 12 miles during the game -- five more than the players, according to data from the U.S. Soccer Federation.
The first fitness test requires speed. A referee runs 40 meters six times. Each of the six sprints needs to be completed in 6.2 seconds for a male referee. He is allowed 1 minute and 30 seconds between each sprint.
The second test examines the referee's stamina in repeated high-intensity runs. He must run 150 meters in 30 seconds, and then must walk 50 meters in 35 seconds. This is repeated 20 times, which equals 10 laps around a track field. Assistant referees have less strict standards.
World Cup referees outrun players [via]