Assume that a given biker is adequately prepared for a century ride, but is not a 'serious competitor'. For the number oriented reader assume a base of 150 to 200 miles per week for the previous couple of months (including some long rides). IOW, reasonably prepared but not competitively prepared.
If this rider is riding a Century ride on rolling terrain (assume less than 5K total climbing), what kind of average HR would be a reasonable expectation (as a percentage of LTHR or average HR on a best effort 30 min TT). Again the rider is trying to attain a best possible total time given the circumstances.
I go about .88 * LTHR for a max average. That means I'll hit LT on several climbs and pedal the downhills. However, the steeper the climbs and the longer and faster the descents, the lower the average HR, all the way down to maybe .82. Higher average HR on flatter, rollier courses.
Interesting, you describe my situation pretty closely for my ride last weekend. I'm not really fast but I do push myself when I'm feeling capable. Here's what my data looked like: (this is for the official 95.6mi course. I rode to and from the course which added 10mi on each end as well.
Elevation gain: 5,161' (4 major climbs, cat 4)
Time: 5:27 (5:13 moving)
HR avg: 152 (83% of my theoretical max of 183).
HR max: 171
Carbon's recommendation looks to be spot on for me. I felt I could have pushed a tiny bit harder but not much. Probably could have maintained 85% max for the whole ride if I had pushed a little more on the flats and rollers.
When I was really fit, and doing lots of centuries and longer rides at a reasonably fast yet comfortable pace, my average HR was about 70% of max. I could cycle 1200K at that sort of average and have energy in the bank if I needed it.
When I returned to Uni and my fitness level dropped off a bit, as did my speed, my average HR went up ... to 80% of max. That was not comfortable anymore and I did not have as much energy in the bank if I needed it.