Building Stamina to test up to be in AC?

I started training myself to run on the treadmill about 2-3 weeks ago, then I got sick and haven't been running in a week... (i think i was pushing myself too hard which made me sick lol) What's the best way to keep on a regimen to work up to p the fitness test? Would running on land be better than running on a treadmill? I'm thinking I'll be taking the physical test early Fall (if that's even possible)... I know I need to be able to run 1.5 miles in 12-13 min.