It is certainly doable for someone dedicated to do three rides a week, two short ones and one weekendride that gets longer and longer during the 6 months. However I am sceptical to people doing it on all kinds of bikes. For fun I rode my rigid MTB 110 miles back from a holiday last summer. I got so much more tired in the hands, arms and neck than I do riding the same route on my roadbike. In december 2006 I decided to do Paris-Brest Paris. That went well 9 months later, it is just a matter of riding enough long slow miles IMO. Do it the french way, have some strong riders up front all the time to set the pace and make it easy for the less experienced.