The absolute fastest way is to use the Teleport Command from the console. Using buff god will make you invincible to radiation and everything else.
Load into any game and press F1
Type out or copy and paste
buff god
teleport 0 0 10000000
You will teleport and the game will complain and say there was an error in red text and then teleport you into a freefall. You will get more than 1000 Kilometers just from that and instantly finish this achievement.
Teleport one more time and you will also get to the bottom of the world for one additional achievement and stop the freefall.
teleport 0 0 0