Marlow's experience in the jungle was of a man used to Western civilization placed suddenly out of his comfort zone, into a world where the instinct is more important and morality is a shifting, grey area. He compares his journey to that of the ancient Romans who landed on the British islands; those people were used to a certain type of world and found what was, to them, a harsh environment where they could not prosper according to their normal routines:
"They were men enough to face the darkness. And perhaps he was cheered by keeping his eye on a chance of promotion to the fleet at Ravenna by-and-by, if he had good friends in Rome and survived the awful climate."
(Conrad, Heart of Darkness, gutenberg.org)
His basic point, then, is that the men who journeyed across the ocean to a harsh place were hardy and strong, both in spirit and in body. They were almost designed to be explorers, people who could put aside their own comfort to find a purpose greater than themselves. Despite the removal of civilization, the Romans continued to search and colonize, and did not descend into madness and anarchy. In that fashion, Marlow believes that men who journeyed into the jungle were similar, but decides that he himself is not of the same strength; he survived, but barely, and admires those men who came out stronger instead of being destroyed.