The best prep for cardio would be core / interval exercise training...
I would spend 30 solid minutes doing exercises such as these:
http://www.youtube.com/profile?user=yelkaim1#p/u/104/Pucl3A68T9M
(Everybody's got their favorite exercises... Personally, I use a lot of the ideas from this guy).
Exercises like this work your core, if not your whole body. A strong body allows you to focus on running instead of supplying oxygen to your weak abs, arms, or back. Make sure you throw in PRT-style pushups / crunches, pullups, flutter kicks, and mountain climbers. If you're feeling particularly motivated, do some 8-count body builders.
Next, do 20-30 minutes of interval training. Assuming you're using a treadmill, jog 1/2 mile at a very brisk pace (preferably nothing less than 8.5 mph). Then, run 1/4 mile as fast as you can (preferably 10 mph+). Repeat this until you've reached 2 miles. Do this running at at least 1 degree incline in an attempt to mimic 'real' running. If you run outside, even better. If you're outside and not sure of distance, jog 3 minutes and go hard for 1. Repeat until you've done at least five 1-minute 'all outs'. At the end, you will have essentially ran 5 minutes at your max speed.
If you did this particular workout 3 times a week (Mo/We/Fr) and simply jogged 3 miles on Tu and Th at a moderate pace (for strength and endurance), you would most likely see noticeable gains within 3 or 4 weeks. You'd increase your Vo2 max and slim up (assuming you don't eat trash). Of course, losing weight allows you to run faster, as well (given you're carrying less weight than you were before -- could you imagine running with a 10lb barbell? Drop 10 and you're sure to shave 10 secs off your mile and a half time).
* I'm not an OCS vet but I was a Marine so I have some experience being killed by DIs.