I was stuck at that sort of pace for a few years. I used to do every run as fast as I could.
In April, after spending last winter running intervals 2 or 3 times a week I discovered it had made no difference at all to the time of my longer runs.
So I did a bit of reading up on things and came to the conclusion my aerobic system was rubbish and I had never spent any time improving it.
So I got myself a heart rate monitor and from about May last year I ran every run at about 65% max heart rate (which turned out to be 11-12 min/mile to start). It was the most frustrating thing ever.
Anyway, to cut a long frustrating story short, in about July I decided to run my local 10k route flat out to see how things were going and did it in 48mins, an improvement of over 4 mins.
I started mixing the training up a bit more and ran a 7min PB at the great north run but I reckon if I had stuck to running slow it could have been a bit better. I should maybe add I was running anywhere between 25-40 slow miles per week.
Anyway, take from that what you will, but if you only ever run flat out try backing right off for a few months and see how it goes. Its actually quite enjoyable running slow once you can get over the shame of being overtaken by everyone!