I just finished this book myself, and what I got out of it, personally, was: most people are delusional about what their physical health and energy levels will be in retirement. Things that sound really fun to you and interesting in your 20's, 30's, and 40's may hold little appeal and/or be too physically uncomfortable for you or downright impossible in your 60's, 70's, and 80's.
The classic example is going backpacking around Europe and staying in youth hostels. Fun in your 20's. Not quite so appealing as you get older, if only because your age is out of sync with the rest of the hostel crowd and its much harder to make spontaneous friendships as you travel.
The whole book made me think of a friend I have known for a few decades. In her 50's and 60's, she frequently wanted to travel but denied herself because she didn't want to "waste" the money. She was always afraid of not having enough money. For that reason, she worked until she was 68. She's got two pensions, Social Security, a paid off house, and hundreds of thousands of dollars in her investment accounts. Now she is in her mid 70's and just doesn't have the energy, stamina, and interest that she used to. She leaves the house to go grocery shopping and walk her dogs. She has more money than she could ever spend in her lifetime, and very little interest in spending it. She has to take her required minimum distribution from her IRA each month because of her age, and she just donates the whole thing to charity.