For the record, went to the gym Friday night, and got my heart-rate up to 170 (and held it there) on the recumbent bike. Had my earphones playing some good-paced rock music. Worked until my out-of-shape leg muscles gave out. So very much able to get a "good" workout in.

And also able to work out so calmly that I can read a book. (Sometimes, with knee issues, that's really all you need -- movement, not vigorous movement.)

I love the versatility. I love the zero impact. (That workout I just did? I haven't set foot in a gym since 2012, and my knee has been aching, lately. I walked in cold, was able to get up to that heart rate, and maintain, with zero knee pain. A treadmill or elliptical would have crippled me if I'd tired that.)