There are lots of times where I've gone on a long writing stint, and not always in the most ergonomical conditions. One of the reasons I like having a wireless keyboard or writing on my laptop is I can switch to a lot of different positions and often if need be. Also I find that if I spend too much time writing with out moving away from the screen the quality goes down noticeably, so I make myself take breaks (walk the dog, make a snack, run an errand) usually come back a little fresher and I've taken some time to do the mental work of writing during my break so that hopefully I actually have something to write. I also find that I get better output if I only write for one or two hours at a time, when I do this I usually try to schedule it out so that I have at least an hour every day to write.