I have an existing well, in a well house, within 400 feet of the pond. I could use this structure, and electric, to run an aerator pump.

I would be tempted to run air line on the surface just for temporary, unless there is a problem with that. Probably create some sort of manifold at the pond to adapt from the long air line to the diffuser/air line setup.

Once I do trenching (later) for electric and whatever, I could run the air line underground in the upper part of the trench.

What material is used for an air line when you run several hundred feet from the pump to the pond?