Yeah I like the PAI idea, although the heart rate ratios light/med/intensive to earn PAI seem to be set the same for me and @sulayr. She's a few years younger and her BPM can seemingly go higher.
Not sure if age should be factored in more, or maybe I'm just looking for excuses :)
Making it the same as the age gradings for running would be a good way to work it.
Makes more sense I think. Otherwise trying to hit the weekly PAI total is not going to beneficial to an older persons health!
Could be detrimental in fact...!