I'm sure it's different for all of us. It's certainly a harder effort for me now at 47 than it was as 27. Back then I could fairly easily spike my heart rate to 190 and now it seems I have to be on the verge of death to get over 170.
The activity makes a difference for me too.
With a heartrate of 120 I'd expect to be putting out around 130-140 watts on my road bike and I could maintain that for hours, but it would feel like work.
Last night I went for a 3 mile run at a slow pace of 10:30/mile and my heart rate was 140-150. I can't run and keep my heart low, even at slow speeds.
In the canoe 120 bpm feels like an effort but a sustainable one. Something I could do for a few hours if I needed to cross a large lake or paddle into a stiff wind. It wouldn't be very fun but I could do it.
The other day when I was playing around with using a bent shaft backwards the stroke mechanics were changed and that same 120 bpm felt like twice the work. I wasn't using my core as much and instead was relying more on the smaller muscles of my arm and shoulders. It required more effort from these small muscles to raise my heart rate and, despite the same heart rate (120) it felt significantly harder (and more painful) than my normal paddling stroke.
How long do you think you could have sustained your effort at 120 bpm?
Alan
