The ending you obtain when you reach the 2000km mark indicates that you reached the starting point of the Darkness, as all you see ahead of you is an oblivion of barren wasteland and monsters. This means you just circumnavigated across the entire world just to reach the point of origin, and reaching the 10000km mark outright says you did. Congratulations: All that remains of life now exists in the last few kilometers that haven't been consumed yet.
In the Dimensional Ruler ending, the hero usurps the Dimensional Ruler and uses their newfound power to save various worlds. But considering that the player can challenge the Darkness in yet another run, it's possible that the previous hero decided to continue the original ruler's actions after becoming bored with their godlike power, seeking another hero to kill them.
