Every other part of the day is interruptible, and most of it gets interrupted. Somebody wants something, a phone buzzes, a teacher starts talking, and your thinking resumes from the beginning each time. Fifteen minutes on a dark pavement is one of the few stretches where a thought runs to the end without interruption.

Treat it like a standing appointment rather than a chore. Same time, no calls, phone left face down at home if you can manage it. Thoughts that need a long run show up there, since it is the only slot available.