Removing remnants of an extrusion operation

[pashman]

Folks, another newbie question. So I have a block, and I'm trying to create a foot-shaped indentation into it. I select a bunch of faces and do an extrusion operation to sink those faces into the shape. Sometimes that does what I want, but then other times it pushes a set of edges below the shape, which I don't want. It's like the cuts lose their resolution. I can grab both faces and dissolve it, but then it leaves a hole in my model. Attached is a case in point. What's the best way to take away from a shape in the way I'm trying?