If you do the 60 mile training ride and are not miserable, go for the century ride, especially since the course is pretty flat. I've found that 2/3 rule works for me when I'm doing normal riding before. With some focused training in advance, I've was tired but comfortable after a 120 mile ride when my previous long one was in the 60 mile range.
If there is a rest stop at 60 miles or so, after that you just have 2 more of those short midweek rides to go with a break in between.