I struggled with the key for a while, but I didn't want to break the key or the ignition switch. Now is the time I sort of wished that I still had the gas tank issue. With one empty tank, I could have killed the engine by swapping to the empty tank, which isn't really a fix for the underlying issue but at least would have stopped the engine.
After a while, as I was getting increasingly worried, I finally stumbled onto the problem. The tilt-wheel mechanism. I shifted the angle of the wheel, and the key released. Whew!