I agree that at first the passages will seem a bit daunting. A lot of it also depends upon how many of the core science courses you've completed up until this point, how long it's been since you've taken them, how well you did, etc. etc.
When I took a diagnostic MCAT I was very worried, but after a Kaplan course and a few practice exams, I really learned to sift out all of the garbage in those passages and focus on the material that was useful.
I'd recommend giving yourself about 4 months time to study and practice. For me, the Kaplan course helped because it forced me to go to class, study, and get used to the testing format.
Remember, the MCAT isn't supposed to be easy. Yes, it is hard, but it is also doable. It requires a lot of work and dedication and if you plan accordingly, you'll be able to do just fine. Don't try to be a hero and take it while you're underprepared. Study smart and very soon those passages really won't seem so difficult anymore.