The problem with redoing OG is that you already know the questions and their solutions so this will not really test your skills on unknown problems. If you don't know the questions well, then it's a good idea to resolve them timed. Try to extract a moral from each problem, not just solve it mechanically. What is the problem testing? Is there a typical trap in it? How to recognize such traps in the future and avoid falling in them?

If you are a high scorer, perhaps it's better to get the Problem Pack I for GMATPrep and train with those or simply go to gmatclub.com and select problems in a certain area with a certain level of difficulty and solve them with the timer on-site. A good book with problems resembling the official style is Princeton Review 1037 GMAT problems.

For stamina, simply repeat GMATPrep tests. I've repeated GMATPrep more than 15 times and I still get new questions. Deinstall and reinstall it if doesn't allow you to repeat the practice tests anymore. Analyze your errors on GMATPrep and think about their causes and how to avoid them. You can also do other CATs like MGMAT or GMAT Club.

For endurance during study I would suggest taking Piracetam (sold on Amazon). It's not a stimulant, just keeps you going. Taken late at night, it may prevent you from falling asleep. On the actual exam you may consider taking a stimulant of your choice to give you stamina and speed but not jitters, Piracetam doesn't work for that.