It doesn’t have to be the most practical or common distance. Just whichever one you enjoy the most in the moment. For example I currently run 5 km a lot at the moment due to lower levels of strength in my calves and the practicality of it (5 km doesn’t take much time out of the day), but I definitely enjoy 10 - 12 km runs a lot more and would like to build up to them again. They give me more time to enjoy the sensation of endurance running and I like the harder mental challenge of maintaining a consistent pace over a longer period.
10 miles.
It’s long enough to be worth going. It’s enough distance to hold a warm-up, cool down, and whatever kind of tempo, fartleks, hills, or whatnot you’d like to toss in the middle. If you’re just taking it easy around town you can see some stuff. And I get done before I get bored.