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.
Well, I should say I live somewhere very flat. I'm certainly not talking about Tour de France style mountain stages. I'm not used to that at all. Also, for 5 hours most people will need to be in a good group/peloton. Doing it solo, like a time trial, is difficult. But I do think most people could get around 6 hours (16.7mph average) within a year of training in sportive conditions (ie. with some drafting, not strict time trial conditions). When I trained my solo average over 50 miles was 18.6mph and I'm not particularly special.
With the additional detail and the numbers you provided, that seems reasonable. Steepness of hills is definitely relevant, haha.