This is my 11th year NOT returning to school. Many recent or near future retirees ask me how could I possibly fill the time. Believe me, there are many times when I wondered how I ever had time to teach.
I tell people worried about facing long, empty days not to worry about it. There's plenty to do. Just take it slow and let things evolve on their own. Maybe start out with a little traveling or home based projects. From there one can volunteer or develop hobbies and interests that weren't possible during their working days. There are those who even work part time at something totally unrelated to what they once did. On top of all of that if you have young grand children nearby, it gets even more interesting.
Finally the nice thing about all this is it doesn't have to be a costly endeavor. There's plenty to do for little or no money.
Try it. You'll love it.