Brainstorm - One man and his dog

A farmer took his dog with him everywhere he went; they were great friends and almost inseparable. One day the farmer had to check on his flock of sheep, which was grazing on pastures 3 miles from the farmhouse. He set-off on the 3 mile walk across the open moorland with the faithful old hound close at his heels.

On arrival at the pasture he checked and rechecked the sheep. Convincing himself that all was well, he started his journey home. The dog knew instinctively where they were going and ran on ahead all the way back to the farmhouse. Finding the farmhouse door closed, the dog ran back to meet the farmer.

When the dog reached the farmer it immediately turned and ran back to the farmhouse. The dog continued running backward and forward between the farmer and the farmhouse until they both reached home together.

Assuming the farmer walks at a speed of 2 miles per hour, and the dog can run at 6 miles per hour,

How far did the dog run on its journey home?

not a clue ...lol