I'm a perpetual over prepared type of person and never even thought of prestudying. Your goal is to get As in your postbacc classes, don't worry about MCAT materials at the moment. Studying for the MCAT mean acing your classes and knowing the materials. That doesn't mean blow off the MCAT. I would check out what type of MCAT materials are available(heard there were good anki decks), prepare your timeline and be ready for changes, and maybe get familiar with the blueprint/outline of what's tested. If you're weak at reading, now is the time to strengthen that because I've seen many posters tank the MCAT on CARS alone.