For solo it is best to have open world instead of normal campaign. Because in an open world where there were no specifically decided goal or end, even the GM is playing/experiencing as much excitement/surprise as the player. Roll for encounter, roll for plot, roll for world development, roll for political move, etc. Just roll for everything and decide later how everything connects together.

Writing a book is when you want to present your gameplay in a more coherent manner. Which a standard campaign fortunately is.