I had some problems relating to the game and this achievement specifically, so hopefully this solution helps.
EDIT:
HawkeyeBarry20 has pointed out that this can be buggy. It's a good idea to have the achievement tracker on-screen to make sure you are making progress. As per their comment, if it stops tracking, progressing the story
can kick-start it again.
As
SwordofWhedon has mentioned in the other solution, this must be done in a single session. This is the cause the of the problem I had. For that reason this is a long and probably over-detailed solution but it relates to a couple of achievements.
This game tracks achievements differently to most (I think). In the archive menu you can find the "Feats" menu which lists each achievement. Somehow, I managed to unlock the achievement on Xbox while the in-game "Feat" tracking was still at 8.6km or so. What this meant is that while I had the achievement, the game didn't think I did.
Jumping ahead to the how, plan to play as much as you can before doing this as all that flight time will count. If you are starting a new session and have made no other progress, I
think this should take around 40-50 minutes.
I recommend proceeding through the story far enough to unlock the Amusement Park location. Fast travel there and turn around. You want to head to the donuts stall on the right (near the drone mission marker, if you know where that is).
This stall has two large ginger bread men on the far side. I found these to be preferable to any other sweet - because they are quite large and, more importantly, flat, they are very easy to land on.
From this stall, simply make large loops back and forth using Beetro. I generally went out towards the wooden gazeebo in the distance and back to the barn amusement in the Amusement Park but essentially you can just fly out until the Beetro meter is around half then turn and fly back.
The gingerbread men only fill about 8/10ths of the Beetro meter, so try to make sure you get back with a little bit left.
Keep flying and restocking Beetro, over and over, until you unlock the achievement. I'm sure there are other locations that work just as well, but this worked for me.
With the achievement unlocked, I
highly recommend checking the feats menu. Make sure the feat for this achievement is unlocked. I'd also suggest completing the next story mission, a side quest or just something else to make sure the feat saves. There is nothing worst than having to repeat this process a second time, believe me...
On the plus side, if you've unlocked all the other achievements and
A Real Achiever hasn't unlocked, completing any feats that haven't registered will unlock it, so at least it's not glitched.
Happy bee-ing!