Use it build yourself up. Please do realize that there is a distinctive difference between MCAT and SAT verbal. The SAT verbal doesn't require a second line of reasoning as does the MCAT. It is more basic in the sense that most questions are main idea or detail related.
The MCAT, or at least the MCATs that are administered today (not really AAMC 3-6) focus much more on your second line of reasoning. At this point most of the dumb people are already weeded out, so the test writers have to take it up a notch. Most of the questions are application, inference, and evaluation questions. You can read the passage and pick out the main idea and details but if you can't REASON around these things, you can't answer the questions. Most of the time the passages are simplistic, but the questions are difficult. This is because 1) you have to reason around them 2) you have have to make sure your reasoning matches up with the test writer's reasoning. This comes with practice.
So go ahead, start with SAT. This with aid you in the global and detail questions (2-3 out of 7 questions on MCAT). From there move on to LSATs or GRE passages (they will help pave the way for the harder reasoning related questions you will face). After that hit the MCAT passages for 3 months straight. You will be more than prepared at this point and will have developed the reasoning needed to mastered the level of critical thinking you need.
Always read actively. Why is this paragraph here? Why does the author mention an authority? If the author posits this point a certain direction, what would he think about the inverse (which a new scenario in the questions will draw upon). Once you think about all these things while still retaining the main idea and where details are located, you're all set. The problem is you only have 7-8 per passage to do so....so it's quite difficult and why most people max around 8-9-10.