As a current cadet at MMI, I can tell you that prepping here significantly increases your chances at an appointment compared to applying your senior year. While I am a CGA self prep here, the only difference in class comparison is that West Point preppers take history instead of physics. The course load you take here is quite rigorous and I can promise you that you will absolutely be challenged academically, however, LTC Bowen (Head of SAP) at Marion does everything he can to help you be the best candidate you can be.
ITC (Intensive Training Cycle): The best way that I can describe this is as a "happy meal" sized BEAST, only being around 3 weeks long, however, I learned a lot about military discipline in that time. This year was a little lighter than it typically is, simply out of an abundance of caution considering COVID mitigations.
Courseload: The list below is the classes that USMA self and AOG preppers have taken this year:
General Chemistry & Lab I and II
English Freshman Composition
Algebra/Trig for Calculus
American History
Military Science I
Physical Training
Extracurriculars: There is a plethora of extracurricular activities that you can get involved with (indicated on MMI's website) however, I cannot stress this enough: Focus on grades before anything else. Do not stretch yourself too thin because most of your time needs to be spent studying to keep up with classes (especially chemistry). I would advise only getting involved in at MOST 3 extracurriculars for first semester. Your first semester grades are very crucial to your chances at an appointment.
If you have any more questions or concerns about MMI, please don't hesitate to DM me.