Answers to this question by some retirement aged folks:
Mark believes that retirement is whatever you want it to be. If it means getting away from an uninspiring job and doing what you have always wanted to do, then I believe. I think you figure out how to make it work financially, even if it means making some acceptable sacrifices. I also believe there is no slowing down. There are so many great, fun things to do you should always be active. Except when you decide you just need to rest for a little bit.
Do something that interests you. It does not have to cost a lot, there are many things to choose from - or try them all.
Keith gave what turned out to an inspiring TEDx Talk on this very subject - check it out at http://www.youtube.com/watch?v=_wq03TOrMQk