Bernadette Mayer's Four Hours
You are missing out if you haven't read the newsletter Subtle Maneuvers by Mason Currey.
In the most recent one, he wrote about the American poet Bernadette Mayer. She lectured at the Poetry Project at St. Mark's Church, and her writing prompts were fabulous.
My favourite was this one:
Write the same poem over and over again, in different forms, until you are weary. Another experiment: Set yourself the task of writing for four hours at a time, perhaps once, twice or seven times a week. Don't stop until hunger and/or fatigue take over. At the very least, always set aside a four-hour period once a month in which to write. This is always possible and will result in one book of poems or prose writing for each year. Then we begin to know something.
The past few weeks, I've written about writing for one or five minutes—short, bite-sized bits to get yourself working.
But what about four hours? Or treat it like a full-time job and work eight hours (hunger and/or fatigue may slow you down).
I used to write for about three hours at coffee shops. I was jittery from coffee and restless from sitting in one spot that long. My ideas would start getting slushy, and I'd have to call it a day. Now I spread it throughout the day, breaking it up with housework and other jobs.
So maybe you start with one minute until you build up your stamina, doubling it over time.
Stamina and endurance are what make you persevere as a writer. Not just over a day but weeks, months, and years.