If you are having trouble hitting those paces on steady runs, you might want to try a workout that I have done in the past when coming back from slow running due to injury: go to the indoor track and get warmed and put on your spikes. Then just run the straights and walk the curves. Act as though you don;t want to run fast, but there is a cable attached to your pelvis that pulls you forward (I run the line and not the lane, pretending it is my rail) at a fast rate. The cable does alll the work and you just move your arms and legs to keep from being dragged. I typically did 20 laps and the workout is not tiring, but is stimulating.