Marlow has experienced a vast change in his character, and his contemplation of the similar effects on the Romans who might have landed on British shores spurs him to tell his story to the others on the boat. He might be telling the story for the first time, and as he gets more involved in the tale it becomes clear that he is unburdening himself, explaining his motivations and justifying his actions in a way that allows him some level of relief from the years of carrying these memories. He starts by saying:
"I don't want to bother you much with what happened to me personally..."
This is perhaps because he originally intends only to tell some of the minor details and superficial aspects, and thus tie it back to his observations on the "darkest places" of the Earth. However, as the trip goes on and nobody stops him, Marlow becomes an orator, an old-fashioned storyteller, reciting by memory an extraordinary experience that truly did change him. At the end, Marlow states:
"Hadn't he said he wanted only justice? But I couldn't. I could not tell her. It would have been too dark -- too dark altogether...."
(Conrad, Heart of Darkness, gutenberg.org)
This is in itself both his justification for hiding Kurtz's true evil from The Intended, and also for his own silence; Marlow is barely able to accept the story himself, but sharing it separates him from it to the extent where he can finally accept how truly "dark" the story is, and how deeply it rooted itself into his inner being.