"개랑 걸어서 가자."
Translation:Let's walk with the dog.
In idiomatic English (at least in the US), to walk a dog means to walk with a dog, usually on a leash, and almost always a pet. While this is a more specific meaning than merely "to walk with a dog", almost all real walking with a dog would qualify. So unless Korean has a different idiom (such as 개를 걸어서 가다 which would be grammatically similar to the English idiom), then in most contexts in would make sense to translate that way.