All of the energy to maintain a glider in steady flight through still
air is derived from gravity - there is no other source of energy.

Potential energy of the glider is converted to kinetic energy in the
form of forward motion in order to overcome drag. In a 1000 lb
sailplane with a 40:1 glide, the wings will develop about 25 lbs of
forward thrust to overcome drag and about 1,000.3 lbs of total lift.
The glide angle is about 1.5 degrees.

I don't understand why people have a problem understanding this - it's
a very similar problem to a ball rolling down an inclined plane.

Just basic physics!

Mike