Yes you shouldn't just plummet like a dart towards the surface, but you gotta go steeper than -5 when you want to go from OC to Glide or it will drop you to normal drive instantly.
So when you approach the OC->Glide transition, make sure you're at like -10 or something. There's a blue scale from -5 down to -.. uh.. something... not too steep... But (near) level flight isn't possible in Glide mode. Maybe that's the problem?
The problem I'm having with that is that if I want to travel a distance along the planet surface, but a bit faster, I climb up and start supercruise (orbital cruise) and then I nose down to get to Glide, but before I manage to get the nose down more than -5 it drops me to normal flight since I'm not going steep enough

Guess I gotta go up a bit higher and dive in a bit again if I wanna do that...