Right now I'm aiming at increasing the distance I run, so speed is less of an issue. As long as I can run a certain distance, that's all I care about. Sometimes I run fast when I feel like it, but if I increase the pace I shorten the amount of time I run, the point being to let the exhilaration I feel at the end of each run carry over to the next day. This is the same sort of tack I find necessary when writing a novel. I stop every day right at the point where I feel I can write more. Do that, and the next day's work goes surprisingly smoothly. I think Ernest Hemingway did something like that. To keep on going, you have to keep up the rhythm. This is the important thing for long-term projects. Once you set the pace, the rest will follow. The problem is getting the flywheel to spin at a set speed--and to get to that point takes as much concentration and effort as you can manage.
From Haruki Murakami, What I Talk About When I Talk About Running: A Memoir (2008) (translated from the Japanese by Philip Gabriel).