The 600 yard distance goes back to the 1960s when they had that "Presidential Fitness Test" mandated to all schools. Everyone had to run the 600 in PE class, along with several other activities or events. If I recollect correct, there was a 50 yard dash, a shuttle run, pull-ups, sit-ups and maybe one or two more. They had some scoring system with each event and you were supposed to achieve a certain fitness level. They did it in the fall, then again in the spring.
For many years, every school kid in the country had a time for 600 yards. The event just sort of carried over and still exists today.