What about exercise?
Well yes, this is one place where I can't argue. I know I write better if I take regular exercise and if you're in the phase of long-haul writing (a novel or non-fiction book, or even a thesis) it is very easy to get depressed about the process and to find you've spent several hours apparently working but actually fiddling around with the same few words.
Exercise that elevates the heart rate breaks through that vicious downward spiral and has a couple of other useful effects as well: it stops the spread that all too many writers find around their middles from long hours at their desks; and it makes you get out of your room and engage with the real world, from which inspiration and insight often come!
How much, how often?
Each writer must find their own routine. But if you write for a living, you need to take seriously the need to be fit, supple and content. Nobody wants to be depressed, nor uncomfortably fat, nor to develop repetitive strain injuries, and avoiding all three is in our own hands (or feet).
When I'm working on fiction I try to stop every hour or so and do something reasonably energetic but when I'm working for a client I break for five minutes every half-hour. If somebody's paying for my words, or my time, it's important that I am at my most productive and that means short, focused writing sessions and regular breaks. If it's my own time I'm wasting, then I can sit and mull over a story as long as I like.