At a normal walking pace, it would take around 20 minutes to walk around the 27-acre square field. If the 27 acres are in any shape more rectangular or irregular than a square, it will take longer to walk around.
Hosseni wrote The Kite Runner to show the world the way that common issues of identity, assimilation and power are carried out in his culture in Afghanistan. He spends a great deal of time developing Amir and showing how he comes to a self-identity by the end of the book.
One answer might be that Hassan haunts Amir and is, in one way or another, the subject of Amir's thoughts for pretty much his whole life. It's also worth mentioning that at the end of the novel, Amir does become a kite runner. The book ends with Amir running a kite for Sohrab. It seems to be another step on his path to redemption that he does for Hassan's son what Hassan used to do for him.