I was thinking about writing as a practice, and as an exercise that builds ability even as you use it. I was thinking about the athletic metaphor and ways in which an understanding of athletic processes can shape understanding of similar aspects of writing. I was also thinking of music as another metaphor.
For both music and athletics, when one begins the exercise/practice, one begins with a warm up. No musician would begin a practice by trying to master the most difficult passage in her/his repertoire. No athlete would try to perform at the highest level without warming up first. But we sit down to write and expect to be able to write instantly.
There are differences between writing and athletics or music that might lead us to reject the need for a warmup: writing is not physical--you can't pull a muscle by writing too hard right away.
This is true, but isn't it also true that if we sit down to write and nothing happens in the first fifteen minutes we get frustrated? What if you had a writing warmup before you start in on the real work--a period in which you began to get used to the general process of putting your ideas in words, and the focus on your writing that is necessary to work on it.
Warmups should be easy; they should be done for the experience of doing them, not for the outcome--a musician plays scales not for the sake of playing the scales, but because the scales are a relatively easy way to begin the process of warming up the muscles and nervous system.
What could a writer use as a warmup? Almost anything--a list of things to work on after your writing period is over, or a list of things to work on during your writing period, or a note to a friend or anything that might be drawing your attention that you can work on for fifteen minutes as an attempt to get settled into the desk chair and into the writing state. You don't want to get absorbed in something that will take too much time, but if there is something distracting you, writing it down as something to return to when you're done with the writing might be one way to put it out of mind for the period of a few hours in which you're writing.
Writing something easy related to what you're working on might be a reasonable warmup, but you don't want it to be something that you're placing too much stock in--the warmup has to be easy to write--something that is free from judgement, something that is about getting started more than it is about production.