There were two things in particular that helped me improve the most.
1) Army Runs. Get a group of people, at least 4, at most 10, and all run single file around a park, lake, whatever. The overall pace for the whole group is slower than usual. The person at the back of the line sprints to the front of the line. Rinse and repeat. Your recovery time is just however long it takes for it to be your turn again. On a 2 mile run, you'll end up doing 20 or more full on sprints.
2) 300 meter sprints. Anyone can sprint for 100 meters, at almost any level of health. 300 meters is a whole different ball game. Look up the FBI training program and see what the time is supposed to be and shoot for that. It was very difficult for me to get under 58 seconds. I can't remember if I ever got to 55 seconds. When we were doing that, we'd go on a run, and do at least 2, 300 meter sprints at the end. We'd sprint down, then walk back. I didn't recover all the fast and would usually have to stand around for a few minutes as well.