imo your body is acclimated to 20, 30, 40 mile rides because you have done it enough times and your system has adapted

venturing longer is simply a matter of doing it and getting used to it

let me preface by saying I'm no long distance champ and this is merely my theory based on my own riding. Like you regularly ride 25-40 miles with no abnormalities.

If I were to venture longer, I'm guessing there would be an adjustment period (i.e. pain) until I had rode the distance enough times for it to become normal