As I've gotten older and
As I've gotten older and slowly more jaded to the concept of intense sustained cardio activity providing relative immunity from heart attacks, I've definitly lowered my internsity and duration of excercise.
Biking and boarding let you cheat a lot, and I stopped wearing my H.R. monitor years ago.
Using the flawed 220 minus your age rule, and working at 80% of that, 130BPM is all that supposedly is necessary for me. I used to bike at 160 bpm for two hours, but lately I had always hovered around 140 or less, and not really payed close enough attention.
I was never an efficient runner, not the ideal build for it and not very trained, so it's no surprise that even at my current 10-10 1/2 minute pace I feel a much more intense cardio workout than I'm used to.
I've wondered if I'm getting closer to G-D while running, especially in very cold or very hot weather, and I certainly hope that I am not!