In simple terms, to improve your time you need to have runs that are farther than goal distance and others that are shorter, but faster than your goal pace over a period of time.
If you are running a time trial, you should get to the point where you can comfortably complete say 3 miles. Also, if your goal pace is say 10 minutes, you should do a number of quarter miles with short rest between in about an 8:00-8:30 pace. Start with 4 and maybe work yourself up to 8 or 10. When you run your time trial, the distance won't seem so long and the speed won't seem so fast.
This works endurance, but also speed, and the result is in the middle. To work on the ability to maintain the higher speed for a longer duration we would look at longer and faster runs and variations on the distance and rest interval of the shorter runs. Weight work would also come into play at this point, and of course there could be any number of issues with proper form and mechanics that could help.