treadmills can help with intervals, but don't rely on them too much.
run 4 miles at one time twice a week slow to medium pace, three miles one day at a medium pace, and two miles fast with intervals the other days. that way you get distance and speed. also sit ups help with endurance. Squats help build speed and explosiveness.
Also, don't neglet the rucksack. I'd advise getting a rather cheap one off eBay or something and rucking 2-4 miles with 20-30 lbs once or twice a week, doing more as you get closer to R Day. Make sure you're following proper rucking technique with good boots, tho. I'm sure there's a website that spells it out, but if you need help, PM me.