I agree @MRex
I was always a fast walker, until I had back problems. It's mostly sorted now, but walking fast still is something I now cannot do. After only a few yards of fast walking, I feel as though my back is coming loose from my hips. I've had to relearn to walk slower.
I can run though, no problems on the tennis court even with the extra twisting and stretching.
The problem with basic surveys of how much people exercise, people can vastly under/over estimate what they're doing.
My parents would say they're really not exercising much nowadays: they generally only play tennis around 4-5 times a week, a long walk is only 15 miles, and when they go out for a cycle ride they rarely go more than 25 miles (since they got a puncture in the middle of nowhere with no mobile phones nearly 20 miles from home), so they're really not active at all...
Whereas my IL thought heavy exercise was when working parking the far end of the car park and walking from the car to the lift daily. (almost 5 minutes walk!)