Interesting discussion. I'm not there yet, but one thing I have observed and made note for myself is that I "think" one needs to practice retirement before they get there.
It appears to me some people think they know what they want to do, get into it, and then fall off of that path for whatever reason - no real interest, too expensive, etc.
It's easy to say....I'm going to "x" when I retire, but if you haven't already started exercising those muscles, it can be a hard thing to launch. Whether its a hobby, golf, part time gig, traveling, or volunteering, what I see is that often people try to jump into it from ground zero and it has no more sticking power than a New Year's resolution.
Without a plan I think it's easy for many to get into a rut from which they can't pull out....and yes, there are plenty of people who are miserable and stagnate in retirement which leads to health issues - both physically and mentally.