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 regular running, 12k. I wouldn’t mind anywhere between 10 and 12k, it just so happens thar I have several regular routes that are around 12k. For racing, half-marathon. Long enough to strategise, but short enough where at my level I don’t need any special considerations regarding nutrition and fuelling.