What is your English level? As you can tell, reading and understanding are the basics of GMAT. If you do not have those two, I have no idea how to help you.
If you really want to attend the program, what you need to do is to work hard. As you said, you have a lot of time to spending on your review. And you still have three months to go. As your situation, I suggest that you should foget about Lsat at this stage. First, learn as many new words as you can (at least all Toefl words). For both tests, an adequate vocabulary is a must. Second, put most of your time on practicing reading, and keep on doing so. In addtion, practicing OG carefully;then, heading for other materials like Kaplan, Princetion, etc. To get a score of 680 is not so easy; but it is not so tough also. Work hard then you will make it. Good luck!