My longest journey in terms of distance within Elite Dangerous was out into the Formedine rift, to the site of the Zurara mega-ship. This was way before there were many large ships, and this was part of a tour where I checked out Elite's creepy and atmospheric generations ships as part of my own Operation Minerva.
It took me weeks to get out there, but when I finally arrived it was quite an experience. There were a number of audio logs (no spoilers!) but most of all the tranquillity of knowing that there was likely not another player for light years around... eerie, peaceful and incredible all in one package.