That's pretty much my fantasy.
I am a runner, and I'm a teacher. Over the years, I've noticed that my training during the school year is pretty inconsistent, and I end up injured at least once or twice. During the summers (aka, my annual pretend-retirements) I'm dead-consistent, and I train like a beast. I put on muscle mass every summer, and I don't get injured. Coincidence? No way! I won't bore you with all the details, but teaching full-time and having children at home makes it pretty hard to stick to a good workout schedule. If nothing else, sheer exhaustion takes over every few weeks.
During the summers, life is awesome. I run more, hike more, cross-train more, eat like a teenaged boy, and sleep as much as I need to. If the legs feel good, I can run longer than planned without throwing everyone's schedules into a tailspin. All of that is pretty much how I envision retirement.
I actually ran into (almost literally) a senior runner a few months ago -- a woman in her 80's whom I see running a few times a week on my way to work. I had admired her dedication and stamina from afar, but when we passed one another, I had to stop and tell her how impressed and envious I was. It was just a quick chat in which she said she was 83 and had run 50+ marathons, but as she turned to go, she said, "The best thing I did for my running was retire and run even more!" I don't even know her name -- she's the Old Runner Lady to me! -- but I want to be just like her when I grow up! ;-)