I am about the same as you. 51 y/o, resting HR in mid 50's, max HR measured at the end of a hard sprint about 196. One of my favorite workouts is some hill climbing intervals where I end up seeing HR at 180+ after each 10 min climb. The 3 min descent gives me enough time to recover and repeat. I don't monitor HR much on the longer rides but I am guessing around 150 or so much of the time.