A large part of writing is thinking. You do not have to have pen in hand or be sitting at your computer to be writing. Imagining a scene in your head is writing whether it is a fictional scene or a nonfiction scene. A professor once told me not to rush my writing. He said thoughts and ideas need time to percolate. Take a walk, sit in your favorite place, go to the gym, and let your thoughts percolate. You will have a more productive writing session when you do sit down to commit words to paper.