Those that can't find the time during the weekdays usually study for a ton of hours on weekends. By a ton, I mean maybe 8-10 hours a day. Since you do not have a time frame that you'd like to get this done by, you can start off by doing 5 Quant and 5 Verbal questions a day - just so you can get familiar with the content. This is a nice approach because if you do this for a long period of time, the content (in particular, the Quant related area) will be far more familiar to you as you take more practice tests and go over more problems.
As for stamina, there's an easy fix. Take as many practice exams as you can. You have GMATPrep, Kaplan CATs,
MGMAT exams, paper tests, the
gmatclub tests, and the list goes on. The more FULL exams you take, your mind will get used to the workload for the 3-4 hour period. This is something that'll come with time. Start taking these exams A MONTH OR SO AFTER you have started your prep. They are an invaluable resource and to use them up too quickly would be wasteful.
You will find a bunch of time mgmt related tips on this site if you search around. Just remember you are supposed to spend about 2 minutes per problem - that's a rough estimate. You can spend 5 minutes on one problem IF AND ONLY IF you are sure you can make it up later down the road. Spending 5 minutes on 2 or 3 problems, however, is not recommended.
Best of luck on your retake.