Quantitative Reasoning

I am signed up to start a Manhattan GMAT course on 1/5/2013. Since its the holidays and work is slow, I wanted to get a head start on the math, since I know this will be my weak spot. I am through Day 8 on the Beat the Gmat study guide, and I am realizing a major slow spot for me is my mental math. I am realizing how much faster I will be able to go by working on knowing a lot of basic math known by memory. My question is, in these 8 days prior to my class beginning, is it a better use of my time to stick with the BTG study guide, knowing that I will be starting my class, or should I focus on mental math drills that gets my brain used to numbers and the relationships they have with one another?