Be miles away
Definition 1 of 1
Pro Tip 1/3Not Literal Distance
The phrase doesn't refer to actual physical distance, but to being distracted or not present mentally. 
During the lecture, Sally was miles away, thinking about her vacation.
Pro Tip 2/3Informal Usage
It's a casual expression, used more in spoken language or informal writing. 
Sorry, I was miles away. What did you just say?
Pro Tip 3/3Conversation Context
Used when someone doesn't respond in a conversation, indicating they're not paying attention. 
You've been quiet, you seem miles away. Everything okay?