I remember doing the Presidential fitness tests in the 60s. I was the second fastest kid in the 600 (or was it 660) yard run in our school in 6th grade. The fastest was a girl who was one second faster than me in the girls race. I think she was sort of an 'age cheat' as she was one of the taller kids in the school. She also had some decent breast development rare for a sixth grader. But she was fast.
Every child was active. No internet, no video games, limited tv channels to choose from. sugary foods were not easily available. (we could only choose chocolate milk in the cafeteria in association with a few holidays). Kids walked up to a mile one way to school. Very few parents drove their kids to school. Even if you took the bus to school you had to walk up to four blocks to get to the school bus stop.
There were a couple outside recess breaks, PE class was still a thing, plus lunch break which were all outdoor play times. After getting home from school we would be outside playing so often that my mom got a special bell she would ring to signal that we would have to come home for dinner.
There were few overweight kids then, and those that were at least tried to be active. There were few specialty sport teams or clubs, and no school sport teams until high school, so all the kids were participating in free form group play.