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.
I enjoy running on my lunch break at work, so a shorter run is good for me. I’ll run 2 miles most days but every few runs I’ll run 5km, just to see if I’m improving.
My average mile time for a 5k. Is like 9 minutes a mile right now, so I’m not an advanced runner by any means.