This might seem obvious but lots and lots of practice! You aren't going to compete in a marathon race without training long distances just like you can't expect to make it through a 5 hour exam without putting in the training time. The more full length practice exams you take, the "easier" it becomes. Well, the MCAT won't ever be easy but it makes tackling the whole exam feel manageable. I suggest treating each section as its own separate exam and to stop reflecting on each section after you finish it. I'm sure those of you who come from competitive sports backgrounds have had coaches preach this over and over again. Only focus on the task you are completing, not on the event you already finished or the events you still have to complete. So as soon as PS isover, forget about it and immediately start thinking about your verbal techniques so your mind prepared for the next section.
On days that you aren't taking a full length my MCAT instructor encouraged my class to study in set blocks to mimic the length of each section. So try spending about 70min at a time working on MCAT, take a 10min break to Facebook stalk then go back to studying for another 60-70min. Also make sure you are really pushing yourself. Once you get tired, push through one more passage or go over that chapter you hate. No excuses, making yourself do the work when you don't feel like it is probably the best way to build up that endurance.
