Yes, I practice straight without breaks. I've never really tried to take a break because my schedule is just that tight... I'm up until 1 usually, 12 on a good day, as it is... and if I take a break I lose my train of thought, like if I'm trying to improvise on something.
Here he goes again with Thomas Mark. My coach's book is "What Every Pianist Needs To Know About The Body," which is known to practically every piano department head of any university in the U.S.
It is a very easy to read handbook, which should be kept by the piano in order to check yourself periodically.
As far as sitting at the piano three hours straight, my comment is just keep it up Bubba, and in no time you will be manifesting all kinds of orthopaedic ailments that you never had before.
The first thing that I do in the morning is a light stretching exercise by hanging myself over the edge of the bed. It is the last thing that I do before I go to sleep.
Do you not realize that the time of day you are practicing is when you spine is the shortest? And, by sitting in one position for that length of time just makes it that much worse.
How much time would it take to get up from the piano every 20 minutes, and then lie on the floor and gently, gently do a slow "snow angel" to loosen yourself up? Please, give it a try.