I have no idea how this can be your answer to the question of energy spent walking upstairs, but nevermind...
M stands for mass. Everything is physical, but I assume you mean that mass must refer to matter. Not so; energy has a mass in itself (as the formula says). So a sufficient amount of energy creates a gravity field.
Hans
Mass and Energy are always equivalent
Now let’s say you have an empty mountain lake.
You now decide to fill up the lake with water that comes from a 1000 meter lower position.
This means
you must use energy to fill it up .
Using energy to that purpose will also mean lose of mass too (in the energy sourse you use)
Even a battery has larger mass when fully charged compared to when it it’s flat. .
So it requires Energy / Mass to move the water a higher position.
Since energy / mass cannot just vanish, it must be “somewhere” so soon you are finish.
And the only logical place it not can be, - is converted to is what you call position energy.
And therefore pretty simply - the energy/mass you have used is now again converted to energy / mass in the filled lake.
Pretty simple the water in the lake have now larger mass /energy compared to before.
There are no naïve hocus pocus smart stupid way of denying this..
There is no way to justify that the lake
not has larger mass/energy now.
It is so logical obvious, the only option in the real world.
You cannot hide Mass/ Energy in a not understood expression either.
The Mass and Energy is physical there, in the mountain lake..
The position energy is the exact same as relativistic energy, and it is always real and physical.
Now let say you can find a technic only to convert the relativist energy to a different kind of energy, - let say to electricity, heat etc. and you will do this on board at a Boing 747, - the aircraft will now be weightless, - simply because you have prevented the aircraft from losing positional energy, and therefore also from falling down to Earth.
Therefore here is a hint to understand what gravity is about.