I do about 2-3 centuries per year. I do 50 mile rides most weekends. I have done solo centuries but that was some time ago. In these days with cell phones, suffering a break down in the boonies is not such a bad thing, you are just a phone call from assistance. The trick with centuries is to not push yourself to hard early on. You can still finish a century after burning yourself out, but it isn't fun. I would suggest knocking off a mile or two per hour off of your shorter ride cruising speed. If you still feel fresh at the 70+ mile point, then you can increase your speed. Centuries are not all that hard if you stay hydrated, eat some food, and ride sensibly. They just take awhile.