When I was doing C25K I would get up the morning after a run and need to stretch my calves and generally wake up my protesting body before venturing painfully down the stairs half a step at a time - firm grip on the handrail, left foot down to the first tread, right foot joins it, left foot down to the second tread...
Without really noticing when it happened I can now run 5k in a respectable time (I judge that as Age Grading over 50%), function for the rest of the day and then get out of bed the next morning as if nothing had happened. And see - I can even smile while I'm running