"A let alone B." Is A more or less than B?

[tomandtish]

For example, which of the following would be correct?

• I can not walk, let alone run.

My instinct is that "let alone" means "not even this one/simpler thing", so the first sentence is correct.

However, I have seen the expression being used the other way around.

The second one is correct, and there are two ways to use it.

I can't do/be this simple thing, so why would I be able to do/be this harder thing.

"I can't even walk, let alone run".

OR I can't believe they are able to do/be this harder thing given that they shouldn't be able to do/be the easier thing.

"I can't believe they are alive, let alone conscious".