You gain progress toward this achievement as your ship sails across the map when you have an active Gold Hoarder voyage running. You can track progress by looking at the in game commendations for the Gold Hoarder faction in your reputation page.
Active Athena voyages do NOT count towards this achievement. Devils Roar Gold Hoarder Voyages do count toward this achievement.
To speed progress make sure you always have a voyage active on when sailing around even if pursuing alternative tasks. For example, if you are battling skeleton ship or sailing to or from a skull fort, you might as well have a voyage down so miles are accumulated. Also, once you tap the last chest and completing a Gold Hoarder voyage you should drop a new voyage immediately even if you plan to sail directly to an outpost.