Heya!
I just tested this. I spawned at 1.2km, 220kmph and I made from 13nm (about 25km) to about 6nm short of the runway. It might be a little higher than a standard GA aircraft, but consider a fighter of the time had a glide ratio of 13-14:1 I am not shocked by a 12km glide to give a 12ish:1 glide ratio.
Remember, idle thrust does not equal no thrust. Even if at idle, it will still generate thrust to move the plane forward. You'll have to turn the engine off completly and redo your test to get the actual glide ratio.