If "Heart of Darkness" follows a hero's motif, then what is the end result? How does Marlow become more than what he is and why?
In many ways, Heart of Darknessis a tale of the loss of innocence in much the same way so many other pieces of literature are with much younger protagonists (think Scout in To Kill a Mockingbird or Huck in The Adventures of Huckleberry Finn). Marlow's journey up the Congo River is much like Kurtz's. Both men are brought face to face with the realities of Imperialism and the devastating consequences it can have on people and entire ways of life. The difference in their journeys of course is that Kurtz is consumed by his loss of innocence; Marlow gains wisdom.
By going into "Hades" in Africa, Marlow transcends the trappings of his society in a spiritual way. Notice at the beginning that he sits on the boat telling his story almost like a Buddha. He also recognizes that Africa is not alone in being a "dark continent"--England too, was once a wilderness that crushed men. The fact that Marlow survives with his mind intact as well as an understanding of what society looks like in reality shows an almost super-human feat in the context of the work.
Marlow sees the senselessness of the exploration and exploitation of the natives and the Congo itself. He also learns what is can do to a man that he admired--Kurtz. I would have to argue that it doesn't follow a hero's motif because Marlow doesn't become more than he is, as the question states. While he may be exposed to "the horrors" of Kurtz's Congo, he still has admiration for the man and can't bring himself to tell The Intended what Kurtz had learned.
You could probably make the argument that Marlow when on a heoric journey--he sought after something (Kurtz), and was transformed by the journey (the journey as more important that his sought-after item), but I don't know if Marlow is changed. The fact that he's relating the story to other--spreading the word of what's really going on, so to speak--could speak of a change on his part.