vampsol,
You'll need to be able to run 2 miles non-stop in less than 16 minutes (male) or 19 minutes (female).
Throughout your time at USMA you will have to pass the Army Physical Fitness Test twice a year. This test includes situps(within 2 minutes), pushups(with 2 minutes) and a 2 mile run. You must meet a minimum standard in each of the categories. If you can't pass the test, you will be separated from USMA.
For 17-21 year old cadets, the minimum standard (max time) for the two miles is about 16 minutes for men and 19 minutes for women.