I did many centuries on a mountain bike! You can do whatever you tell yourself you can do. I was actually averaging 15-something too. To be honest, my average actually went down when I switched to the road bike but I think it was because I wasn't used to that type of bike.

So ride what works for you! I honestly didn't mind the long rides on the mountain bike. I did put road tires on it though, I would recommend that at least.