I wanted to join the Marines, and still do, but they wont take me because of a shoulder injury in high school from when I was wrestling.
I recommend long slow distance training, supplemented by interval sprints; the long slow distance (7-9 miles at a bout a 9-10 minute mile pace at least 4 times a week) will build superior endurance in your heart, lungs, diaphragm, core, and legs. The interval sprints (200 yards sprint, 200 yards walk, for 8 laps around the track) on days you dont do long slow distance will build intense strength in all of the above areas, and together with the improved endurance they'll bring your time down monumentally. When I entered my RSS, I ran a 23 minute 3 miles, but as I continued to run (following the program I just laid out for you) I slowed my running pace to a formation run pace, and ran long distances (ended up hitting 16 miles twice a week at one point in time, and running no less than 9 on days I wasn't running 16). On days I was tired I would just do sprints, and within 2 months I dropped to sub 18.
I haven't ran in a while though; I stopped running once I found out my BUMED got denied. But if you follow that, or somethign similar depending on your current level of fitness, your time will improve.