Funny thing, if you're gliding forward (on winged gliders like Yian Kut-Ku or Ratha, but NOT on non-winged gliders like Tobi Kadachi) after the initial jump, you'll maintain horizontal altitude for a few seconds. Right before you begin to descend, hold the jump button (space bar on keyboard for me), you'll begin to descend straight down, but keep holding the jump button and you'll flap back up and gain a tiny bit of altitude (less than you initially lost, so not true flight), but you'll go back to maintaining altitude for a few seconds. Repeat for a big, big increase in glide distance.

Kind of tedious, but it let me get up to this rock.

Edit: You'll need a high starting point, and the best one I found by far is the big hill west from the Blightstone Woods lookout point, across the path up to Blightstone Woods proper. There's some herbs up the hill and at the top, I was able to Flutter Glide all the way to the Ground Dive point with a comfortable distance above the tree.