Do you take breaks? Remember that you have break time during the exam - between each block you can get up, walk around, have a snack, whatever. It might be good to do some "practice tests" using whatever Qbank you're using, just to work on the timing of your breaks. Try taking a short break between each block, or maybe it will be better for you to push through the first few blocks without any breaks, then take most of your break time between the last blocks.
Really, 5 hours is pretty good, and you should expect to be feeling a little drained after 5 hours of just about any activity, but you do need to be able to get through to the end. You don't want to rush through the exam and make extra mistakes because you didn't give yourself time to double check.