The AAMC and Kaplan practice exams both have the 'mark' feature (called 'guess' on the AAMC tests), as well as the 'review' feature, which takes you to a page that displays all the problem numbers, and indicates whether you have answered a particular question, left it blank, or marked it. Make these features your friends. Get in the habit of marking questions that are taking longer than one minute to answer, and move on. If you do so, you should have plenty of time at the end of each section to sit and think about the more difficult questions, with the confidence that comes from knowing you have already answered all of the easier ones. Remember, the MCAT is a numbers game, and you don't get more points for the difficult ones, so save them for last if you can.
If you get in the habit of pacing yourself this way, then it will be second nature on exam day.