It is extremely important that you sit for at least 3.5 hours at a stretch as that will be representative of what you will face on the final day. However, I will recommend mixing the topics to kill the boredom. For example, mix Cr with Problem solving and Sentence correction with Data sufficiency.
Also, I can share from my experiences of studying in a top 10 school: Be prepared to face the grind and studying for as many as 16 hours a day. And that too building excel models, reading 100 page long case studies etc. So, I am sure you will have to pull yourself out of the comfort zone. You can google for “Black November at Darden” and you will know what I am describing here.
We emphasize on building the stamina ahead of time in our classes here and I can clearly see that it is very important.
Moreover, I want to ask: You mentioned 20-30 questions in 2-3 hours? Is that correct? I can see that speed if definitely a problem there ( unless you are taking too long to analyze). I feel that you should be able to do 50 problems with thorough analysis in 3 hours( not sure if I am missing something in your post).
Regards
PythaGURUS Team