I did a century last July - this was a group ride with a posted speed of 13-14mph which is slower than I like to ride but I wanted it as a training ride just go get a century in as well as cover new terrain. The ride leader kept the pace as advertised. This was a very easy ride, I did not feel pushed to stay with a pack early and I did not get tired early. There were a lot of breaks and a lot of converstaion while riding, we were not huffing and puffing. This ride sounds like it could be a fun all day event, take your time and don't worry about the time on the bike.
BTW - next weekend I will do my last century of the year, it will most likely be slow (13mph) as I will ride with a group who are slow riders, again - not worried about elapsed time, just want to enjoy this ride. Now with that said - I did my fast century in early Sept, I like to push for faster speeds but this ride is not the time for it as it is more a social event.