The words "less" and "fewer" are applied to non-discrete and discrete values respectively. So in lines in supermarkets it should be "ten items or fewer", rather than "less". With regard to miles, you can have part of a mile, so it's five miles or less. With regard to a Planck distance you can't actually have less, so you need to have fewer of them not less.
There are fewer than 5 ^ 38 Planke lengths in a five miles.
There are less than 27,000 feet in five miles.