Stories come to us as wraiths requiring precise embodiments. Running seems to allow me, ideally, an expanded consciousness in which I can envision what I'm writing as a film or a dream. I rarely invent at the typewriter but recall what I've experienced. I don't use a word processor but write in longhand, at considerable length. (Again, I know: writers are crazy.)
By the time I come to type out my writing formally, I've envisioned it repeatedly. I've never thought of writing as the mere arrangement of words on the page but as the attempted embodiment of a vision: a complex of emotions, raw experience.
The effort of memorable art is to evoke in the reader or spectator emotions appropriate to that effort. Running is a meditation; more practicably it allows me to scroll through, in my mind's eye, the pages I've just written, proofreading for errors and improvements.
My method is one of continuous revision. While writing a long novel, every day I loop back to earlier sections to rewrite, in order to maintain a consistent, fluid voice. When I write the final two or three chapters of a novel, I write them simultaneously with the rewriting of the opening, so that, ideally at least, the novel is like a river uniformly flowing, each passage concurrent with all the others.