I've just discovered that the MS bike tour that I'm signed up for is minimum 135k, optional 155, so close to a century. Fortunately, it's in mid-August, so I've got 8 months to train.
Heart rate monitors can be great if you know your zones. There are several different methods for determining them. I'm not sure which one was used to find mine, two winters ago when I signed on for a progressive spin class put on by a local cycling club, but I hope they haven't changed much in two years. The idea of the class was to do our training in 18 week cycles where we spent 6 weeks in the zone2-3 range, 6 weeks where we added in more and more zone 4 intervals, then 6 in which we continued with zone 4 intervals and threw in some all-out sprints in zone 5. Probably more complicated than most people need to go for recreational riding.