Extruding a "channel" into a complex shape .. sort of a 3D inset

Not sure how to describe this properly … start maybe with a cube and tear its corner vertices into all kinds of odd directions, subdivide it, pull them all into more directions … create a totally oddly shaped object. You could also imagine a natural face or some other irregular shape … a rock.

Now use two edgeloops around it … this will form the “channel” I want to extrude. I want the extrusion “belt” thus created to be exactly the same height everywhere … sort of like an inset is also always the same distance from the edges you started with.

How do I do that? How do I extrude, so all extruded heights remain the same even in totally weird shapes?

You could try extruding (E) and cancel the translation (esc or right mouse button) and then alt+S. Other thing to try would be to use inset instead of extrude, and slide the inset to 0 and then hold down ctrl to add depth instead.

With images and a .blend file of course.

you can also Try Bevel (CTRL+B) or Inset Faces (I-Key)