When we talk about energy, we talk about energy transfers.
Steady level flight, constant speed:
The state of energy of the airplane remains constant (if we neglect the mass reducion due to fuel burn).
However, to maintain that state of energy you need to burn fuel, right? Otherwise you would slowdown or descend. This is because drag removes energy from the airplane, transfering it to the air in several forms (moving it and heating it, mainly). You replace that energy with the energy coming from the engines (much of which is wasted in the air as well, by the way).
There are many energy transfers, between the airplane and the air, and in both senses. The fuel burnt allows you to counter the drag caused by your airspeed. Your airspeed makes the lift that keeps your altitude constant.
The net result, in this case, I think it would be the fuel burnt. That is what keeps you up there for as long as you want.
In the case of a glider maintaining constant altitude and speed, the energy lost by drag has to come from the rising air, entirely.
nice thread