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 like 8-10k. Far enough to get me in the right frame of mind and quick enough to fit in most days I need to. I do enjoy 20k on a Sunday morning plenty of time to catch up on a couple of podcasts.