Lehninger is very good, but, personally, I am a big fan of Lubert Stryer. I think Stryer is the greatest. He has such a straightforward, common-sense way of explaining and his love of the knowledge shines through. You don't need a recent edition. Unlike molecular biology, the core knowledge base in biochemistry has held pretty steady for the past thirty years. I would go back to Stryer's 2nd or 3rd edition. The 2nd edition is a compact wonder, although the illustrations in the 3rd are a little better. Alternatively you could get a back edition of his Short Course in Biochemistry which I think was an attempt to recapture the magic of those early editions. Lippincott's biochemistry review would be a good supplement. You definitely need to go above and beyond MCAT materials in learning this material. You have a real challenge ahead of you.
You don't need to study the whole book but you do need to know everything up to and including Amino Acid Degradation and the Urea Cycle for the new MCAT. After that, you can skim. You need to have an idea of the shape of 2nd semester just to be sophisticated, but you don't need to memorize the biosynthesis of phenylalanine etc. The second semester is full of long, drawn-out biosyntheses. Skim that.
It's really important you start right away and take this seriously. Sophistication with biochemistry is the single most important predictor of success on the new exam. Learn that 400 pages of Stryer (or Lehninger) so that you could open your textbook at any place, immediately recognize the discussion and give a synopsis. Take it one step at a time. Multiple cycles through the material are more important than one long grind.