[yomly]

Well a brisk walk (from memory) is about 15 minutes a mile.

So people under the age of 10 and over 60 probably start having a non-negligible numbers of people who can't complete a mile in 15 mins.

And then you have an obesity + overweight rating of 57.6% in the USA according to Wikipedia (and this may not cover people who are not overweight but physically fit) and you can start seeing how maybe not everyone can do even a 15 minute mile walk.

For the record I know BMI is a bit of a weak metric for covering obesity given that anyone "swole" will have a very high BMI...