Yeah, it can be tough getting back into academics! You want to build up your endurance slowly. Each day, do a little bit more, just as if you were training to run a marathon. On the GMAT, you'll have to sit and work for a long time--that's one of the great challenges of the test! However, the test never forces you to sit for more than 75 minutes at a time, so you don't need to push much past that in your study. That doesn't mean that you should only study an hour a day, but that you should give yourself short breaks to get up and move, have a snack, etc.
It can also be helpful to mix up the *type* of studying that you do. Your sessions should include all of the following types of study/practice:
*New instructional content (reading strategy guides, learning concepts)
*Practice problems (mostly from the
OG)
*Review of practice problems (spend more time here than on doing new problems)
*Memorization/drill (formulas, multiplication facts, idioms, etc. This should be done in small doses, 15-20 min. max.)
*You may also want to start off with easier warmup problems--material you've already conquered, or drills on fundamentals. (There are a lot of these in our Foundations of GMAT Math book.)
If you mix up these different kinds of study, you are less likely to get worn out on one thing. (After all, you use your brain all day; it's just hard to use it in this one focused way for a long period, especially if you are trying to do so after a long day at work.) It also helps to mix quant & verbal, rather than sticking to one area for your whole session. Start with the material that's harder for you, and then do the easier one for "dessert."
So, if you break your study into many pieces, and make sure that every study session has several of these pieces, you don't have to work very long on any one thing to end up with a few hours of study under your belt. This should get easier as you go along and get more comfortable with the material. It takes a lot of mental energy to pay attention to things that are new and challenging. As more aspects of the test become comfortable and routine, you should find the experience of studying less exhausting.