Distance puzzle

This is one of my favourite puzzles. Read it in 'Hard to solve math puzzles' by Derrick Niederman. I won't be reciting it word-by-word though

Captain A is leading a huge battalion marching towards their headquarters, which is 20 miles away. The battalion is so huge that it measured 20 miles in length. At this time, as they kept marching at a constant speed, soldier X, the man in the last row had to pass on a message to his leader (in person). He started running towards the leader who is in the first row of the battalion, met him before the leader could reach the headquarters, and again ran back to his position. When he did so, the leader reached the headquarters, the rest would follow.

So, what do you think is the total distance covered by the soldier when he reached the headquarters?
(You may ignore the time taken to convey the message)