Yes, the title is heavily inspired by the similar PF2 question about the Fly spell, with the difference that air walking is more walking than flying.
While Air Walk is on...
The subject can tread on air as if walking on solid ground.
Last session Alice, an air-walking PC with a good reach, managed to get near enough to enemy spellcaster Bob to threathen him.
Bob, obviously uncomfortable with Alice being this close, wanted to remove her from combat and cast the Slumber hex on her.
In my (and Bob's) mind, Alice would have fallen asleep on the spot, in mid-air, on the same air-feel-like-terrain ground she was previously standing on.
In the rest of the party's mind, Alice is not treading/walking on the air anymore, so she falls through air, takes some damage from the fall and wakes up, still threatening Bob and still going to full attack him next turn unless he moves away.
Some case could be made that if the need to be treading/walking is literal, then stopping and just standing should make you fall, but I think nobody rules air walk that way, and once you step on some air-ground it keeps supporting you until you take the following step, even if you stop there for hours.
So, witch is it? (Pun intended)