Last 45 mins are stalemate

Yes, you have to build stamina, but you don't have to do it by studying in 4 hour blocks. Think about how athletes train for a marathon. They don't run 26 miles everyday, but they do run almost every day in varying distances. You can mimic the same training regimen through timed sets. Whenever you practice GMAT problems they should be in timed sets of varying lengths, as short as 10 minutes and as long as a full hour. In your situation it would be helpful to do these timed sets back to back - like 20-30 minutes of quant followed by 20-30 minutes of verbal and you could even through in a few IR questions at the beginning of the session. This way your mind gets accustomed to needing to be totally focused for blocks of time with short breaks in between. On test day you are just lengthening the blocks a bit.

The second thing you need to do is maximize your breaks between section. You need to completely free your mind of what you just did and refresh yourself. That includes getting a snack (granola/power bars and sports drinks work great), get your blood flowing (walking around, even a few jumping jacks) and doing some good breathing (deep breaths can be refreshing during the exam as well). You only have 8 minutes but you want that 8 minutes to be as refreshing for your mind and body as possible - it can pay big dividends.

Lastly, prepare your body and mind beforehand. Get plenty of rest the night before the exam and the night before the night before the exam. Eat healthy meals before the exam. Visualize yourself doing well on the exam and stay positive throughout.

Best of luck,
KW