I'm retired, now 67, started at 59. So far I've learned a new occupation (public insurance adjuster), written a couple of books, and done home improvement after home improvement. Recently I built a boat. I also started a blog on one of my ongoing interests, the American Civil War. I've also had a new knee installed, a nerve relocated in my elbow and, most recently, a stent installed in the widowmaker artery. My wife and I are also traveling, although it's pretty much in week-long tourist-mode chunks rather than my ideal goal of four to six months in a foreign country at a whack. That may still happen, the issue is her health, which requires fairly frequent medical involvement. But it's still good, it's just life shaped to the realities but not surrendered to them.
Retirement is whatever you want.
On the book? I can be more helpful there.
Set a minimum goal for whenever you sit down to write, a goal you can't get out of your seat without meeting. Mine was 2,000 words, but that included reading the last 2,000 words and editing them, to pull back the hammer on that day's writing, so to speak, before pulling the trigger on the new goal. If you read, and edit, and then start, it actually goes faster, because you've refreshed your mind on the big picture in your head and can pick up the story a lot faster. Also, if you're hardcore INTP, envisioning the entire project each day is non-motivational, while envisioning just the next 2,000 words (or whatever) is.
I hasten to note that this was a pretty easy pattern for me, as a newspaper journalist for my entire life. All human experience is reduced to a daily output of less than 2,000 words for journalists. :-) Your mileage may vary.
I think it is helpful to opt for looking forward to retirement as an opportunity. Mine, due to the national economy and the pathetic condition of the newspaper industry, came a bit sooner than I'd have liked, so I had to deal with frustration for a couple of years. And I have to say this forum was helpful in that, even though my specific issues were never addressed on the forum. It is simply therapeutic to be among people who accept me for the introverted, sardonic bastard that I am. :-)