How is Marlowe's quest like a journey of a hero? To me, Marlowe seemed somewhat uncaring of the situation of colonial Africa.
Marlow's journey is heroic because he continues his search for Kurtz in spite of many obstacles. He rejects the brutality of colonial Africa just as Kurtz surrenders to his own "heart of darkness" which includes greed, brutality and rejection of civilized society. Instead Marlow makes a difficult choice. He also realizes that he has his own "heart of darkness" and that society is indeed brutal. But to reject his mission would provide no integrity. Marlow's realization, acceptance, and constant struggle to maintain integrity is truly heroic considering the monstrous obstacles he encounters. This makes Marlow as much of a hero as Beowulf.
Heroes in mythology and classical epics undertake a journey in order for some sort of self discovery. In "Heart of Darkness" Marlowe undertakes his journey because he has the potential to be exactly like Kurtz and allow the horrors of colonialism in Africa to consume his humanity.
Marlowe's descriptions of what he encounters have a tone of detachment so that the reader is never sure whether he will be overcome by the situation and fall into the same savagery as Kurtz.
By the end of the novel (and Marlowe's journey of self discovery) he is appalled by what he has seen and realises that he could easily become Kurtz. So, in effect, he learns from Kurtz's decline into savagery.