Try and run a mile or so every other day until you go. Its not nearly enough to prepare you for anything, but at least you wont get slack - you cant gain much in 12 days, but you sure as hell can lose alot.
If you drink or smoke, youre going to have to stop anyway so put yourself through "mini-detox" now. Drink lots and lots of water every day - if there is yellow in your urine, drink more.
When you get down there, the only things that anyone can really tell you is 1.) Keep your mouth shut. 2.) Square yourself away quickly and encourage those around you to do the same. 3.) Dont quit. Youll be amazed at the punishment your body can take. 4.) As hard as it seems when youre there, you will look back upon it fondly. 5.) Dont worry... the rest of your service wont even remotely resemble basic.
Thank you for your service - good luck!