Also, don't look at breaks as a major unplug or nothing at all. Our brains work in 90-120 minute cycles (similar to our sleep cycles) and so building in short breaks (even only 5-10 minutes) can be incredibly refreshing and make you more productive than if you cranked through a whole day.
The book is no BS either; the authors have helped Fortune 500 leaders and professional athletes rise to the top of their game (like helping a tennis player become #1 in the World).
Happy to talk more some time as I'm just wrapping the book up and starting to apply some of the lessons from it. It's helping me understand why for instance my most productive hour of my week is the hour after i keep home from my soccer games and how to have more heavy execution sessions like it.
Don't feel guilty. It's about working smart and hard.
But part of business (and the advantage for so many successful companies) is creative thinking and the ability to see an opportunity that others don't. Stepping back and 'turning off' is critical to the creative thinking process: it's the 'incubation' stage that leads to 'illumination' or the Aha-moment: http://blogs.hbr.org/schwartz/2011/11/how-to-think-creativel...
So next time you feel like a slacker for having a beer with friends, know that you might actually be on the verge of a breakthrough.
If you love what you do and are motivated to do that thing for such long hours, more power to you!