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.
For racing, half’s are my favorite, despite not racing them often. Long enough to be hard, long enough to not be a pain cave race, but don’t destroy you for weeks to recover.
For a day to day run, 8 miles/12km is a good sweet spot for me. A good duration, but not too long, not much recovery needed. Can just go do it.