Go to a track where you can accurately measure distance. Once round is 400 mtrs. Work out the pace you need to be at, then go at that pace for a minute, recover a minute and do that for about 20 minutes. Every time you go try and go a bit further at the pace but keep recovery at same time. you'll build it up then. Good luck and let us know if you make it.
I agree with SB, speed work and Fartleks will help to increase your speed over that distance fairly quickly. The other way, would be to try running at the required pace, (after warming up properly) and seeing how far you get, keep repeating until you reach 110% of the required distance in the time. This will allow some margin for the fact that the army may not measure accurately. Try both and see which works for you.
Comments
Jonny
Best of luck!
CN