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 am pretty overweight. I can run up to about 10km, and my heart/breathing is fine, it’s just my legs that start to give up. I’ve gone for three runs in the last 8 days, longest being 4.7km which I was pretty sore after. The other two were 3.7 and 3.8. I felt much better after those.