If the year were 1971 instead of 2021, you would not have to ask. The answer is 600 yards (548.64 metres). I held my school record in 600 yards for less than 2 hours. I was a middle distance runner. A 400m/400m specialist broke my school record before the ink dried. Up until 1970's, 600 yard was raced, 400m guys versus 800m guys, closely fought battles.
Seven big names, 600 yards, 1) Lon Myers, 74.5, a 440 yard man (1st sub-50 440 yard man), 2)Thomas Burke, 71.0, a sprinter, 3) Mel Sheppard, 70.8, a 400m to 1500m man, 4) Douglas Lowe, 70.4, an 800m man, 5) Martin McGrady, 67.6i, a 400m man, 6) Mark Everett, 67.53i, an 800m man, 7) Butch Reynolds, 66.87, a 400m man.