To contribute a real world perspective...
I once did a demonstration of a DC-8-72 where I reduced thrust to idle from FL240 @ 74 NM out to a straight-in. Field elevation: 1288'. I was able to land without touching power, within 5 kts of on speed at the threshold just by managing energy with gear and flap extension timing (there are no speed brakes on a DC-8, BTW) The landing weight was ~205,000# on a 350,000# TOGW limit.
When flying the Classic 747, one must adjust idle power descent point depending on TOD weight if planning a min fuel burn descent without ATC interference. The heavier the weight, the longer the glide, and the lower the weight, the shorter the glide. The difference in a no wind situation glide between landing weights of 630,000# and 380,000# could be ~25-30 NM. Airplanes with a lesser delta would obviously be affected by weight to a much smaller degree.
From a small airplane standpoint, gliding distance improved very noticeably when I added flap gap seals to my 185. Descent planning to a VFR traffic pattern required a re-evaluation due to the resulting drag reduction.
Engage in speedbrake-free energy management without a green arc or dot. It's an enjoyable game.