Minor nitpick with your first sentence: I do not believe "anyone with a bit of training" is an accurate way to describe what is required for pedaling a bicycle at 20 mph for 5 solid hours.

At a time when I was in better shape than the average person, the most I ever worked up to was 40 miles, and it was brutal. In my entire middle-aged life, I have met exactly one person who believably claimed to have done 100 miles in a day. I'm sure it's easier than many/most people think, but not 'anyone' easy, with just 'a bit' of training.