This applies mostly to non-fiction writers, but it can also apply to fiction. When you choose to write for a long period it is important to choose a topic that you will be able to write at length about without getting really bored.
Even if you are writing a story, if you do not care about what happened to prehistoric vampire fish, then do not write a story about prehistoric vampire fish. Or whatever.
A subject that you are interested in will enable you to conduct better research and ultimately, produce better content.