In my prime 30 years ago as a runner I could run a 39 minute 10K and my marathon PR is 3:06. I am not super faster runner the fastest mile I ever ran was 5:41. I do have endurance as I can keep it up for long time. The point is that once in my prime I wore a persons heart rate monitor, back then they were just getting started with using them and I never did. Well I remember even then at age 30 the highest I could get my HR was in the around 185.
As I mention my running is messed up with runner's dystonia so I cannot run all out for a mile or so. If I hold on to treadmill rails and push I can get my heart rate to maybe around 150 but to me it is like I am trying to run an all out mile. I think on the bike the thing I seemed to do is I can keep going back and pushing. I have no trouble riding with riders much younger than me, unless of course they are the local race studs. This morning I rode 54 miles in 2:57 miles and my HR was 106 average and I hit 142 on a steep hill. Todays ride had 1214 ft elevation according to Garmin. I don't know how that works but a bit more hills in this ride than normally.