For several months I've been going by the Borg Scale of Perceived Exertion, which uses 6-20. I like it because the descriptions of the various effort levels give me a way to compare my cycling efforts to other activities; things that I do more than cycling.
Re: century training: the Garmin can show me how much time I've spent in which heart rate zones on any ride. Since I began checking that data, it's become plain that I've been spending the lion's share of my ride time in the "moderate" zone. I decided it's time to start jamming it up to "brisk" sometimes.
When I tried it this morning, it took well over a minute for my HR to climb into the "brisk" range. Good grief, am I really as fit as that??