If you want to race at a fast pace, you need to train at a fast pace several days a week. Interval training where you alternate fast running with jogging or walking is a very effective form of speed work.
This is part of my 3 WEEK RUNNING PROGRAM!
Run the 400 meters at about the pace you would run in a mile. Walk or jog between each repeat 7 times. Although the best venue for speedwork of this sort is on a 400-meter track, these workouts can be done on the road or on trails, either by using measured courses or by running hard approximately the length of time you would run a 400 on the track. For instance, if you normally run 400s in 90 seconds, do fast reps at that length of time and don't worry about distance.