Chin ups are not allowed - just pull ups. Like people have said before, practice is the only way to improve on pull ups. Keep on at it, hopefully 3+ times a week, and do let your arms recover after a particularly hard workout. One thing that is not mentioned is the issue of weight. I'm sure not an issue for most candidates to USAFA, but losing five pounds of baggage (hard running is good at burning calories, as well as improving cardio) while continuing to practice pull ups will yield excellent results. Just a little extra bit of weight can have a large detrimental effect on pull ups.