run away with the idea
To take a fantastical or unrealistic idea as something that is definitely true or destined to happen. His book appeared on a local bestseller list, and now he's run away with the idea of being a famous author. Local tabloids are running away with the idea of the mayor being a part of a big conspiracy.
run aˈway with the idea/notion
(spoken) believe something that is not true: Don’t run away with the idea that you’re going to be famous just because you’ve appeared on television once.