How to get better at long distance running?

I think I'm going to join the Army. I kow they run LOTS and I'm really not a very good runner. The longest I have ever jogged/ran is 2 miles but this was about 2 years ago after soccer season when I was in very good shape. Now I can only run about 3/4 of a mile without stopping to walk. Please give me as many tips as possible!