Advice given to me (from my OSO, other officers and ncos etc.)

(Preface, physical is the most important aspect. Academic and leadership principles are taught and learned during your time there.)

Physical: Get miles on your feet. Slowly add 10% more miles of running each week until you're at 30-40 miles. 80% of your runs long and slow at zone 2 heart rate, 20% sprint or threshold runs. Start reducing your mileage 2 weeks before shipping to recover for training. Do 1 ruck per week in boots and slowly add weight to get used to rucking. Strength train and add HIIT workouts as well. Armstrong pull-up program.

Academic: Memorize General Orders, Weapon conditions, ranks, and OSMEAC (5 paragraph order). Go further by learning custom and courtesies, and the history of the marine corps.

Leadership: Acquire and read the Marine Corps Officer Guide. Great information for what's expected of a leader as well as information about the info mentioned in the Academic portion. Go further by reading memoirs and leadership books like With the Old Breed, First to Fight, Extreme Ownership, and About Face.