I'm going to write this blog post in twenty minutes.
Almost anything you have planned in your day, especially those things that you'd rather not do, can be done in twenty minutes or less if you treat it as a mild act of meditation.
Since everything is an act of balance, you need only balance the two opposing methods to productivity. On one side you have the masculine approach, that is, to enter problem-solving mode, wherein one can focus on one thing only until that thing is resolved or completed. On the other (feminine) side of things, you can do this thing called "mult-tasking," which doesn't really exist, as the brain can not do more than one thing at a time. So what multi-tasking really entails is the ability to switch quickly between tasks without worrying whether they have reached completion.
Marrying these two energies just means that you have to completely focus on one thing for a preset amount of time, and be able to leave it unfinished when your time is up.
Defining completion in a work of fiction is about the moment you reach the exhale, the contraction of the lungs. Stories are the act of respiration, particularly novels. They are sequences of build-up and catharsis.
So you might not want to leave your work until you have finished a particular scene or cycle. Or even a line. Yet most novelists (myself included) will advise you to leave a writing session while you still have a solid idea of what happens next. This sets you up for the next writing session with confidence; it gives you a place to begin and, more importantly, it helps settle that pesky problem of continuity.
Don't worry about what part of your piece you want to finish writing today. Rather, make a decision about how much time you're willing to spend on it. It will help you be present with the work. It will teach you about your own process, your abilities, and about what the work needs. You may find that you work through some pieces faster than others. You'll have a better understanding of what the act of writing actually means for you.
Well, it hasn't been twenty minutes, but I think I've made my point here. I hope it's helpful.