Sometimes I mix some walking into my run. There is benefit to spending a certain amount of time on the legs, or covering a certain amount of distance, regardless of the effort level or aerobic zone in which it occurs. Bones, tendons, ligaments, muscles -- all strengthen in response to the stress of being used.
Yet at my age ( cough - mid-50s - cough ) I am extremely wary lest injury cause a setback. The idea of doing like Prefontaine and going all out all the time is a recipe for injury.
So yesterday, for example, I did my 5 mile Sunday "long run" by setting the interval timer on my watch to repeat and alternate between a three minute interval and a one minute interval. Run three, walk one. That worked for where I happened to be condition-wise that day. My overall time ended up being right where the McMillan Running training pace thing advised me to be.
Another successful training week!
Plus, I get out and see the pretty poppies!