A few weeks back, I posted about the definition and importance of repetition (R) workouts. Here are a few sample workouts you can do. Best place to do these is at a track, so its flat and you know exactly how far the distances are. If you take it out on the road, just be sure to find as flat a road as you can. Any incline or decline will mess with your times you should be hitting.
Make sure you have a good aerobic running base before trying these workouts. Be sure to start each workout with a 10-15 minute warm-up (easy pace). Follow it up with a 10-15 minute cool-down. (Each * is a different workout).