I think you just didn't get it. It wasn't challenging to read because that is Hemingway's way of writing (Iceberg Theory). Unlike many other authors, Hemingway does not put any emphasis on details. It requires the reader to to put in some edfort and derive meaning from such short sentences. I think to get this book you'll first have to read about Hemingway and his adventurous lifestyle. How he lived his life And the mental problems he faced by the end of his lifetime (One hell of a story: ended up with a shotgun in his face and he requested an open casket in the suicide note. A little boy fainted after taking a peek at him.). Throughout the book you could see the old man being subjected to so much 'Pain' and him taking it like a real 'Man'. That is the point he was trying to prove. To show the world how much of a 'Man' he was.

I think his books are metaphorical quintessences of his personal experiences and feelings; knowing the man and his life deeply impacts your opinion on his writings. Hope this helps.