We say "My mind was running away with itself" or "His imagination is running wild". Briefly it means that you are daydreaming about a lot of things that may or may not happen. For instance, you buy a lottery ticket and you start to think of all the things you can buy with the money; the winner hasn't even been announced! :D A more negative situation would be "My husband didn't come home last night, my imagination was running wild." (She was thinking of all the negative reasons why he might not have come home.)