whoa, slow down there my friend. I think its admirable that someone who hasn't even started med school yet is already worried about the USMLE, but I assure you, it's a little premature. Review books in general, and First Aid in particular are just that....review. First Aid contains short phrases, catch words, and brief pneumonics that are designed to make you go "oh yeah", and remember a complex concept or a highly detailed subject. They are NOT designed for primary, first-time learning. No first year could read a section out of the pathology part of First Aid and know what the hell its talking about. Honestly, even after two years of classes and reading First Aid 5-6 times all the way through in the past 6 months for the Step 1 I just took, and there were still some things in there I don't fully understand.
just to clarify, I don't mean to discourage you, in fact I want prevent you from being discouraged by starting your "review" prematurely. Trust me, you can wait till 6 months before Step 1 to start studying like everyone else, and then you'll go through all the review books you can stand. If you are highly motivated, I would suggest going through the relevant parts of First Aid as part of studying for a final, but don't be too hard on yourself. I know I underestimated the pace that med school would keep, you simply can't do all the "extra prep" you planned on.
Anyway, good luck with everything, I'm sure you'll be great. I'll leave with a saying we gave our first years:
"The most protective measure a first year med student possesses is ignorance of what's to come"