Been there, done that - it sucked.
I feel your pain. Rice, potatoes, oatmeal (not the instant stuff), scrape together some cash, if you can, to buy yourself some cheap protein supplements (they'll taste like crap, but ounce for ounce, they're much cheaper than meat) also tuna (not too often though, too much tuna= too much ocean pollution).
You want to be able to succeed on course, so train for that, don't train to be a soldier, the instructors are going to assist you in that. Note - I start basic next Monday, so I'm not an authority, I'm just a guy who's managed to defeat a lousy physique. I'm no superman.
Use your body-weight, push-ups, situps, running, pull-ups, chin-ups.... these things will help, when you build a solid base, THEN you have to tools you need to pack on some serious muscle. It's really basic... there are a few threads here that deal with specific workouts. Start slow if you have to, work up to it.
Running in itself isn't going to bulk you up, but it helps develop core muscles, and will help your cardio.
Look up paracowboy's post: "Shin splints, Blisters and PT" there's some good stuff in there.