I think he was asking why you weren't training while not in service?
As far as what to do, read the site. There's plenty of programs, diets and what not on here. Beginners section is a good place to start to keep it nice and simple in terms of eating and training.
Since you have 4 weeks, you need to be running 5-6 times a week.
Start out running the distance of the test, if that's easy, start running more mileage. Again since you're short on time, HIIT might be more beneficial as it's shown to illicit quicker, albeit not necessarily the best long term results. Since you just need to pass the test and you're not trying to be a competitive distance runner, a mix of sprints as someone suggested and actual mileage will be your best bet.
Eat enough to recover. As much as you are skin and bones, this will actually help some on endurance running as that's less weight you have to carry around. I'd say pass the test first, then worry about actually getting in shape, ironic I know, but that's the military testing protocols for you.