Noting hb2011's comment about being a more heavyweight athlete and a woman, I was one of those in the last century (!) working on my upper body strength before going on active duty. Had the running thing down, but needed to build up other skills.
I worked on mastering the flexed arm hang first, which builds endurance and mental stamina, supplemented by weight training for upper body, arms, back and chest, then proceeded to pull-ups. Now past the mid-century mark, I can still do them - only because I never stopped doing them, along with push-ups and sit-ups. The answer is to roll out of the rack in the morning and invest 15 minutes in a routine designed to move you along the path toward routine mastery of these physical skills - EVERY DAY, or most days. As Nike says, "just do it," recognizing it's an incremental process and once you get there, you have to maintain. If you haven't been able to do a pull-up, it's not something you can get handled in cram sessions a few days before you head off.
Take a look at the flexed arm hang, which I think is what female Marines do instead of the pull-up. It's a good one to work on to build confidence and strength, male or female. Harder than you might think, if you have never done one. For women, hand strength is sometimes an issue for bar exercises, so get one of those spring grip exercisers and build up your grip. And, most gyms have those assisted pull-up machines where you can build up to your full body weight for the pull.
http://www.topendsports.com/testing/tests/flexed-arm-hang-pft.htm
http://www.ehow.com/how_2159587_flexedarm-hang-marine-corps-pft.html
There are many reputable online fitness training sources which offer tips and techniques for improving pull-up skills. Just do it... and good luck!