I often fly (in an ultralight) at ~30 mph, and above 2000 feet or so you just feel like you're standing still.

Possibly like just hanging in the air; the rate of change would be so gradual. And I imagine it's immensely quiet in the cockpit, too, with just the sound of the wind, if any.

prevailing winds probably about that fast, generating most of the groundspeed going east, and it sounds like maybe not even feasible to travel across the country east to west.