You can practice stamina even when not doing CATs or practice questions. When you sit down to do a 2-hour study session, plan out EVERYthing. Then GO for one hour. Don't check your email. Don't talk to anyone. Don't even get up to get something to eat or to go to the bathroom. Pretend it's the test!
Then take a 10-minute break and do it again for another hour.
Then take a substantial break (at least one hour) before studying any more that day. (You don't want to tire yourself out too much while studying - if you do, then your brain won't be able to make good memories, so your study will be wasted.)
What do you feel like when doing those questions? Do you start to lose concentration and find yourself thinking about something other than the problem in front of you right now? Do you start to get tired of the test and wish it were over already?
You may want to read this and think about how you can change the way you study and the processes you use to answer questions in order to help you build mental stamina:http://www.manhattangmat.com/blog/index ... you-crazy/
Finally, don't neglect the physical. Make sure you have had something to eat and drink somewhat recently. When you do take breaks, get up and walk around, stretch, do some jumping jacks. Etc.