Dutchman Adri Hartveld (his website finished 5th in the Dutch Cross Country Championships in 1984 (photo below) and won the Dutch marathon title in 1986. He has worked as a physiotherapist in the UK since 1982 and, during that time, has assessed hundreds of runners with overuse injuries. As part of the assessment, Adri looked at both the athletes’ biomechanics and their training history.He also researched various training ideas and methodologies, including easy itnerval training.
 
Please scroll down below the photo to read a lecture that he gave at the University of Staffordshire in 2017.

From this lecture we learn that recovery can take about a week after a heavy effort (note: after an extreme effort (marathon) or several heavy workouts shortly after each other it can take longer!). We also learn that the best super-compensation takes place when after the heavy effort a light training session is completed once every two days.