As a 60-year-old whose HR zones seem to be about the same as when I was 45…maybe 5 bpm lower now, at most.
I’m way slower. It’s maddening sometimes. I ran some 1:50 half marathons off the bike in triathlons in those days. I can barely run a rested 10k at that pace now, and I’m in better running shape!
That said, I don’t have any trouble doing the same mix of *relative* intensities that I ever did. Everything is just slower.