My numbers are not like your numbers. My RHR is around 50 these days.
I find I settle at around 110-130bpm for a steady 15-20 mph cruise.
140-150 for a motivated effort (300-350 watts) 20-25 mph
and maybe 170 for short high intensity (400-500 watt) bits like a steep hill 10-20% climb or 25+mph mile or 3 then it immediatly drops back to some nominal level at the next stop light & makes the average look terrible.
I'd like to get back to the days where I could achieve a 95% HR intensity on a bicycle, outside of a controlled environment, but I just don't think my legs can put that kind of draw on my cardio system anymore 'cause my cardio system adapted & something always interrupts the effort. 5-10 hours a week of 80-90% intensity on a fluid trainer building strength & capacity over the winter will do that.
I find that over training jello legs makes it difficult to put a decent load on your system as well. Give it 2 or 3 days then go out & give it a good motivated effort & see how your heart responds. My guess is you'll be able to make it work harder, but still not reach super high intensities.
Big cardio is nice. Also as a runner, you probably have well conditioned & engaged core muscles your riding partners may not & tons of mitochondrial adaption throughout your body to withstand the high intensity of running many of your riding partners may not.
What is their VO2 max? What is yours? I'll bet you take in bunches more oxygen at a lower intensity effort a whole lot easier than they do.
I wouldn't think too much of it. Your RHR is definitly on the low end, but you can still get a solid 100 point spread out of the activity.
Congrats, in my book, you are very, very fit.
(and I should probably go back to running 5-10 miles a week for the intensity.
)
(Stand by for a post about statins, or some other medication coming soon.)