I agree with ennay. I think people underestimate how difficult it is to increase your speed. Going up that much in a few months is quite an accomplishment. You should be really proud of yourself!
I think mixing it up is the best way to get better. Sometimes I run endurance runs, sometimes I run longer sprints for 1 minutes, sometimes I run all out sprints for 15 seconds. When I started running sprints (also after seeing it on BL) I was running at nowhere near what they were. Even now, I see some of the contestants doing things thirty pounds heavier than me that I can't do. It's not realistic for someone without that kind of aggressive training, medical attention, and supervision. What they do is amazing, but you can't compare yourself to that. What you have done as a regular individual is amazing.
So maybe you run sprints at 6.0 right now, but in two weeks maybe you decide to push it to 6.5 and do it for a little less time. And then that gets easy and you push it to 6.7. I started running my all out sprints 3.0mph
slower than I do now. It takes time. Just find ways to keep challenging yourself and focus on making yourself a little uncomfortable no matter what you do. It's easy to fall into a comfortable pace (especially when you're running outside) but by pushing yourself passed that just a little you're challenging yourself a lot more.
And now I'm rambling so I'll stop. Hope some of that helped!
Edit: I was also going to say I think Ashley was at 9 or 10mph max. Sunshine was running at 12.