Truth. Nothing is a greater spurn to creativity (cyclic mental exertion) than time away focusing on cyclic bodily exertion.
Usually I'm wrong though and taking a break would be a much better use of my time. Walking, biking, noodling on my guitar, or even going for a drive all seem to work for me.
My only reliable "hack" is to have a workout/exercise partner and/or trainer so it becomes a mutual shared commitment.