Thanks so much for the reply nadia0410, it boosted my confidence.
The Kaplan class I took offers 'The Ultimate Practice Test'. Tomorrow I take the GMAT for real at the same test center I'll take it two weeks from now. I think it goes without saying how freaked out I am but I'm going to try and go in with a clear head.
I'm really wondering how much mental strain has to do with how I perform. Today I put in 11 hours at work and during tonight's practice session finishing up the OG I did worse than usual on the 50 problems I completed. I think I do better after 48 hours of not working. I hope this holds true when I take the test on a Monday after a weekend.
Also, and this seemed weird to me - I hit the 620 score after I took the practice test completely. Up until that test I had always skipped the essays. I'm wondering if finally writing each essay played at least some part in my dramatically higher score. Kind of like warming up before a race? What do you guys think?