To miss by a mile is a great expression.
Imagine that you are trying to hit a baseball with a baseball bat. Your friend throws the ball at you...and you swing...BUT you miss.
Imagine that someone is video taping you playing baseball, and that night you watch the video and you see yourself miss the ball. As you are watching you see that the ball is above your head when you try to swing for it at your waist. Your bat was no where near the ball. There was about half a meter between the ball and the bat. You missed the ball; you missed it by a mile.
It just means when you miss something that you had NO chance of succeeding and the chances of you succeeding were very low.
For example, you missed the bullís eye. You missed it by a mile (imagine that you threw the dart and it didn't even hit the dart board).
If a pilot is trying to land a plane but doesn't land on the runway...he missed the runway by a mile.
Also note that it is more common to say "he missed it by a mile" than to say "he missed the runway by a mile". So you would mention the object before.
He didn't even hit the dartboard. He missed it by a mile.
He didn't even land on the runway. He missed it by a mile.
He didn't stop in front of the house. He missed ti by a mile.
Does that make sense? If you need more examples just let me know. Or someone else will help you out for sure. :)
Have a great day!
English Grammar. What Would You Like to Know?