I remember when I first looked at the Kaplan book at Borders before i had taken a single class. I thought, WOW! this is outrageous, no way in hell can i know all this stuff!
Here is what I can tell you from my experience....
Dont be overwhelmed. You will learn 90% of that book in your classes. Rather than memorizing the kaplan book later, you will be simply refreshing your memory with the kaplan book. Also consider the DAT will not ask questions in the amount of detail you will learn in classes. Thats generally speaking, there are a few detailed questions on every test that seperate the 20's from the 24's (especially in Bio). So most of the DAT will seem pretty straight forward after you have studied the material in class. IMO, if you ace your classes you will probably be scoring 15-17 right away. With a lot of practice with sample questions (and learning shortcuts/DAT tricks) you will bump your score from there.
Just take it one step at a time! BTW, start practicing your math reading, and PAT skill now! You'll be glad you did later.
Things you can do now:
-Subscribe to American Scientific and start reading articles
-Buy a Barrons SAT math workbook and refresh your skills
-Look at Barrons PAT guide and get familiar with the rules
-Secure/save funds for your application process
-Kick butt in your classes
-Take kaplan 3-4 months before you plan on taking DAT.
You'll be there in no time. Good luck