fitness test

Hi everyone. New to the site it looks great. Right iv got an armed forces fitness test coming up pretty soon. Press-ups, situps, swimming and running. I'v been training for it for a couple of months now and everything is going fine apart from I'v got to run 3 miles in 24 mins. 8 min miles right. I'm finding this so extremley difficult. I'm running 5 times a week with 2 rest days. I can do it in the required time (only just) but the problem is my legs get so tired so quickly and they start to feel like jelly. my lungs are fine i think i could keep going for ages with my lungs its just my legs. So the problem is lactic acid right? I'v read elsewere that interval training is good for this. Any other methods? Load up on carbs? Any info appreciated. thanks

When I went to boot camp our PT went from not being able to run 1 mile to running 6 in 8 weeks. Pretty cool right? We did our PT like this. 6 days a week PT. Every other day muscle failure. On regular days we would do push ups for 2 minutes then rest while counting off our buddies then 2 minutes for sit ups and then rest while holding feet and counting off our buddies. After that we would jog 2-6 whatever our ability was at the time. On failure days we would do push ups for 2 minutes 1.5 minutes 1 minute .5 minutes. The same was done for sit ups and remember that we would rest for the same amount of time for each exercise as we did the exercise. So if you did the push ups for 1.5 minutes you would rest for 1.5. After this was done on failure we would sprint a mile. Of course you're not going to be able to sprint a mile but that wasn't the point. I would say it's a lot like interval training only we didn't have a set time to walk and a set time to start back up. The 6 miles was practically nothing and at the end of the 6 miles I felt like I could run forever.