6000 Feet
Is actually quite easy to get, the hard part is getting all of the upgrades so that you can hit 6000 feet within 20 or so days (whichever ending, I think, that doesn't say "it was about time").
Basically, think physics (but also, if you notice how the glider works):
When you go up, your speed will go down due to a lot of causes, but, that gives you altitude.
With enough altitude (you can and or should use the rockets right as you come off to do this), and enough speed, say, 200 MPH, find the "sweet spot" where you either don't lose speed, or you gain speed (just a tiny bit) while altitude still falls relatively slowly. That way you'll glide for maybe the first 5000 feet before having to refresh yourself, which is the (relatively) harder part.
To refresh yourself, you're going to have to shoot up before your speed is too low (around 100 mph or so), but then make sure you get enough altitude - without making the speed go under, say, 50 MPH. That way, you can glide for a few hundred more meters to reach your goal.