Here's what I did: I got everything else done with the exception of completing the final mission and a few achievements and challenges. Yet, I still was at 77 percent for the achievement. So, I put the quest marker on the final mission in the middle of the map and flew under the floating island. Then I flew back and forth from one end of the map to the other. When the game made me turn around I flew the other way. It was a good 3,000 meters each way, which netted me 2 percent toward the achievement. When I finally made it to 99 percent I flew to the mission and then flew in circles until I got the achievement. You can even complete other achievements while working on this one.
The following achievements can be gained while working on this achievement: