There's all kinds of different ways you can run to improve different ways. Being in the army for 5 years has definitely taught me that

Fartlek (Cardio Training): Jog at a very slow pace for a little bit, until your heart rate gets up, then sprint for a little while. Then go back to the slow pace again. Repeat for a while. If you're running in a town, you could sprint for 1-2 blocks then slow jog for 1-2 blocks. Alternate back and forth for a mile or two.
30-60s or 60-120s (Cardio Training): Like the fartlek, jog to get your heart rate up. Then you sprint as fast as you can for 30 seconds. Then slow jog for 60 seconds. Keep alternating until you've done it 10-15 times.
Hill Climb (Leg Strength): Simply running up and down a hilly area will help improve the strength in your legs.
Distance (Endurance): Just running for 5 miles at a fair pace (9-10min/mile) will help increase your ability to run long distances without tiring.
The army's test involves running 2 miles in a certain amount of time. If you're an 18-21 yr old male, you're maximum time would be 15:56. 21-27 yr olds get 16:36. I don't know the maximums beyond that (not that old yet

) but if you're female you add 2-2.5 mins to the max. Just a proven physiological fact: The average female's muscle system will make them run 2 mins slower than the average male's muscle system will allow. (I add that fact in case you were wondering why females get more time do to their runs)
Of course, I've seen females that will run 2 miles in about 12 minutes (My best time is 14:30) So just because that's the maximum time they're allowed, don't just shoot for that

Any questions, just ask.
-C