Try some fartlek or speedwork training. Last friday I did (for the first time ever) 7 miles alternating 1 mile slow jog (7mph) with 1 mile run (8.5-9mph), just to see what would happen. My splits worked out about the same right through the run (same for each slow bit, and same for each fast bit) but my overall time was as good as I usually manage for a 10k (6.2miles).
I've been stuck in a bit of a rut lately - I can run long distances easily at a 'comfortable' pace, but found I had trouble if I tried to increase my pace (i.e. increase it over the whole distance). I'm going to try some of the speedwork tips I've found on the site and see how I go.