There is an optimal mental state for creative work called "flow" where you become completely absorbed in the task at hand and lose a sense of time and place. And if you want to maximize your writing productivity, you'll want to learn about this flow state--and develop the ability to enter it on demand.
To get started, I highly recommend Mihaly Czikszentmihalyi's book Flow for an excellent and thorough description of the mental state, how to master it, and how to put it to use.
It typically takes 15-20 minutes of focused effort to enter a flow state. Thus if you use daily 30 minute minimum writing sessions, be sure to leave those sessions open-ended. You never want to waste an opportunity to work in this highly productive mental state, and it would be a crime to have to wrap up your session just as you enter it.
Why? Because when you're in a state of flow, your writing quality and output is far higher than typical. You are so deeply engrossed in your work that you easily tune out distractions. More importantly, you'll also tune out negative self-judgment, a congenital problem for most writers.
I've found that my productivity while in a flow state is so far above my average productivity that the output from one flow session often exceeds the combined output of several other writing sessions. And these sessions are so satisfying that I'm always happy to trade five or six--or even twenty--frustrating writing sessions for just one where I experience flow.