According to that "rule of thumb" that a person's max heart rate will be about 220-age, my own max HR would be 163. Last year, I did a couple gravel races (>4 hrs each) in which my HR
averaged 163+.
Those were pretty epic, at least for me.
zacster , I had the same experience on Seattle-to-Portland. Got to Centralia (100 miles) in <5 hours, and took about 6:10 to cover the next 100 miles to Portland -- and only did the second half that fast because a very kind man pulled me for the last 30 miles.