What has worked for me is a grouping of long/slow endurance runs (like 4-5 miles at a pace slower than your goal run time... aim for a conversational pace) and intervals (doing a certain amount of laps like .5, 1, 2, etc. around the track at or faster than my goal pace). The long runs build endurance and the intervals build speed. As you improve, try to throw some tempo runs in. Tempo runs are comfortably hard. In other words, you should be breathing hard a bit, but not going all out. This gets your body used to maintaining a decent pace over a longer distance.