When I was studying for the MCAT, I read the Examkrackers advice that said, "Take a break, or your brain will go ahead and take one while you are trying to take the test." I still think this is excellent advice.
Step 1 is bad, but you are fuelled by adrenaline and fear. Step 2 CK and CS are brutally long and boring. My personal strategy for the computer tests is a break after every block, a chocolate treat after every block, and quick walk outside after every block. This is only possible at certain test centers, of course; our local one is excellent.
Other people like the blast-through-and-get-done-before-I-die-of-boredom approach.
You definitely need to build endurance by doing a lot of practice questions, just like training for an endurance race.