Well it really depends upon your goal. If you want to run the furthest you can possibly run, and thus train your body for endurance capacity, then keep on running!
The idea is to stress your body beyond the point that it is currently used to. If you can already run 6km, your body is well equipped to handle that, so continue to push yourself outside of your comfort zone and go for 7, 8, 9 or even 10km!
Here's an article I think you may like to read which deals with this concept:
http://www.aminoz.com.au/the-art-of-adaptation-a-165.html
