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.