I've been able to average 8:30 - 9:00minute-ish a mile for 10 K for probably 3 years now. And, on occasion, I go farther than 10K, stretching to 10 - 12 miles sometimes.

At those distances, my speed doesn't change much. But if I try to run a 10K at a noticeably faster pace (like down closer to 8min/mile), it is very difficult and I really feel it.

I guess what I am getting at is that, in my experience, it is much more difficult to increase your pace than it is to increase your distance. I bet you can keep your pace and go much farther without too much difficult training.