It isn't just stretching. It is also form and not putting on to many Ks on to soon. Beginning runners always always always make the mistake of increasing their distance to much and running to fast.
You want to improve your cardio, I'm telling you here and now that unless you're missing half of your lung your "breath" will improve faster than your legs can keep up.
The Iliotibial Band is usually the first to get injured. http://www.time-to-run.com/injuries/thebig5/itb.htm
Now, this may sound silly but I respect your decision to join. I respect that you are smart enough to know to plan ahead and train in advance. Now please believe me when I say you'll be prepaid if you start now (now-ish), start slowly and build slowly.
I said this before, I know people who can do the elliptical/bike/rollerblade hard for an hour and a bit but literally are sucking wind after 10-20 minutes of running. Running is the best for you, it is also the hardest on your body. So be smart (like you are being) and go get someone you know who runs to show you or take a class. Go to "the running room" and I know they will have resources there for you. (I hate to pitch commercial places, but I do believe they are the best) Remember, listen to your body. If you train to hard you will injure yourself and you'll be far worse off than if you just sat on the sofa watching the weather channel. (Me definition of hell)
Oh yea, as a side note. You'll likely be given your boots before you go to boot camp. I'd recommend breaking them in while you can ... but do not run in them
There are allot of threads on what you'll need for your boots (insoles) but I will tell you right now, get things called boot bands. (little stretchy things that you can fold your pants under) It will save you more hassles and stress than any physical training you'll do. Also, if you're like me and you got tonnes and tonnes of used kit, wash everything - even if it looks clean. Grab a sharpie and label EVERYTHING you were given. Yes that includes your underwear. Your unit will show you how (usually Lastname and last three digits of your number)
If you've got a good unit, which if memory serves I think you do, you should be able to have the Cpl who is in charge of looking after the recruits show you how to dress properly. (and BTW you'll not get the beret right, just don't take the lining out until you're done BIQ) And oh yea, before I forget don't stress to much. From my experience the BMQ course is 70% mental exercise. The CF wants you to succeed, the instructors are there to bring you up to the level where you can pass. (only if they believe you'll never meet that level will they punt you out)
once again, many many many tips here on bmq. Search function is awesome. But the biggest one I can give is "Listen to what your instructors say". Honestly, 90% of everthing they care about they will give you lots of heads up on. (closely followed by never lie, mark all of your kit, always look busy, if you're given a 5 minute break take it, enjoy it but be done in 3) Oh yea, address people by their rank. Don't call NCMs/NCOs Sir, you get two freebies then you're getting strips torn off of you.