It's a medium speed - not too slow indeed. In fact it is very close to my training speed. However it's very strange that you can't complete a 10 Km run. Do you feel any special after the 8 Km run?
Better use cross training such cycling, hiking to strength your feet.
I used to hike a lot before I focus on running. Now I could complete a HM without too much difficulties. Four years ago I could hardly complete a 4 Km distance run.