First and foremost: Set realistic goals. Is this book going to change your life? No. After publication, you will not be a different fifty-plus-year-old person. You will be pretty similar to the person you were before, only this fifty-plus-year-old person has written a book. So ask yourself: What are you hoping to get out of the experience?
Yes, it is possible to have a very successful writing career later in life—and doing something new later in one’s career helps to keep you young.