I agree to some extent about the 9 - 5 thing being in many ways bad.
I do exactly what you describe a few days a week.
The problem is that this makes it very hard to properly communicate with other people in your job setting. Nobody knows reliably when other people will be available, whether its for a meeting, or just to get some little bit of information. It works great if you are on a self directed task that lasts for the whole "day" and nobody needs you for anything. It sucks if you need 3 or 4 people to meet to discuss something.
I've also found that, with creator type people, they almost universally prefer larger blocks of contiguous work time in order to be efficient. So interrupting those work hours with other things reduces efficiency.
Clearly, sitting for 8 hours is not healthy. Personally, I could not possibly stand for 8 hours without my feet hurting horribly. Even 4 hours would kill me.
The ideal physical work envirionment would probably consist of a mix of walking, running, standing, squating, etc. IE, just like a human would have experienced while procuring food, defecating, mating, etc all day, in pre-civilization times (insert "evolutionary time" or "garden of eden" depending on your belief).
Thats hard to do for many lines of work these days (computer programmer, writer, artist, etc).