Anyone else having a problem flying higher?  I can't ever seem to each the alternate paths via gliders (coins by the ferris wheel in water park, bridge area in bone dry dunes, etc)

I hold back on the control stick and just fall like a rock.  I think I fall faster when I do this than if I don't move the stick at all.

Any thoughts?

you have to boost to hit some of the high spots. The coins lined up infrontt of the jumps give you boost as well as jumping at the apex of the ramp as well.

As far as I can see from gameplay video's it does seem a little harder to go higher, but I'm sure it's doable.

In Bone Dry Dunes and Mount Wario I have found that the easiest consistent way to always gain enough height is to start gliding by the edge that is on the highest altitude (which is to the right on these particular levels) and to never move the stick up or down.

Only case where I move the stick down is if I'm very close to reach the coins at the end of Water Park, in which case I do it on the last second. Because as you said doing it early causes you to lose speed and, eventually, height.

It's hard getting to some of those short cuts but not impossible. You need to have a good boost to reach certain areas.

Yeah, I guess I am just used to MK7's glider which was easy to get some good lift on

Yeah they basically put a rock on top of the gliders, so all those cool tricks and techniques from MK7 are no more.

I believe the weight of the character plays a part in gliding speed, as does the glider itself.

That'd be Bone Dry Dunes, and that's the one part of that course I actually like! 

Try using a different glider. I used to use the paranoid until I felt like I wasn't getting enough height so then I started using the cloud. Thee standard glider is great for speed and the paranoid is balanced while the cloud actually helps u get higher.