Most often, when I tell people that I write for a living, they say, "I've got this great idea for a book. I"m gonna write it someday." Somehow, that sentence seems to minimize the hours of study and energy most writers put in every day to just keep their writing careers going.
Many people don't understand that having a great idea is just the start. I don't want to scoff at it. Having an idea is an important part of writing a book, but there's so much more. If you have a great idea for a book and don't have the time to write it, you needn't ignore it altogether. While you're waiting for the time offered by retirement or vacation or whatever, start working on the thinking part of developing your book.
One great way to start turning your idea into a book is to create a timeline of the plot. Start at the beginning. List the date and time and what happens at the beginning of the book. Then, in very small steps, list each scene/interaction of each character in the book.
- Saturday, morning, John walks into his brother's apartment and finds his brother dead on the floor.
- Saturday, morning, John calls the police.
- Saturday, noon, John is taken in for questioning, etc.
Slowly, over time, your list will grow until you have a complete chronological list of each action of one or more characters in the book. When you finally have that block of time to write, you'll be that much farther ahead and will have a greater appreciation of the complexities of writing.