Thanks guys, this makes more sense now.

The error I was making is assuming that the most "efficient" cadence would produce the lowest heart rate. When in fact "efficiency" from a cardiovascular standpoint has little to do with it.

Spreggy's response makes a lot of sense - paraphrasing: the higher the cadence, the more aerobic cycling becomes, and aerobic activity can be sustained for a longer period of time.

So to digest all this and try to answer my own question - if I'm training for a fast century, it sounds like I should train at a cadence that puts me just barely under my anaerobic threshold...and I should ride the century at that same cadence?

Thanks again!