Never do this. Take all your pre-requisites and then plan to do a comprehensive review of everything. I mean, if you're an average person with average abilities, you really won't understand much from the study materials for the MCAT. I do encourage reviewing what you know, as suggested above...but what you don't know, you'll waste your time. Those review books are meant to fill the gaps in what you may have trouble recalling or understanding; they never fulfill completely what you should know about something. So, coming from an average person with average abilities, I suggest you don't do this. Relax now and then worry later. It's not worth stressing over the MCAT now. I did that and messed up in so many ways, which I'm not going to mention in this thread.
HOWEVER, there is one thing you could start doing...reading things you don't like! Prepare for verbal now...not the sciences. Verbal is what bugs everyone!