I took the exam years ago, (when it was "only" five hours), but found that the time flew by, and that staying focused was not a problem. I did take a couple of mini-breaks to stretch and relax.
Because I have some adult ADD, I find that caffeine keeps me focused. It doesn't work for everyone, but it's a suggestion. If it makes you jittery, stick with water!
If you've studied the CPT and ICD-9 guidelines, have a good general knowledge of A&P and terminology, have done several practice exams and have read and understood the sequencing and modifier rules, you have prepared as much as you possibly can. Try not to let your nerves rattle you. It's a challenging exam, but not impossible.
The best bit of advice I was given (and I followed) was to not study the night before, get a good night's sleep, and know that you've done all you can to do be successful. It must have worked, because I passed.
Most of all, good luck!