"Idę" points to continuous, present action whereas "chodzę" reffers to present simple form. "Chodzę" could be continuous but it's informal and often goes with "sobie" - "chodzę sobie", where "sobie" doesn't mean much, it can be roughly translated to "just". (I'm just walking around).
Go! = "Ruszaj!", "Idź!" (commonly used imperative forms). Let's go! = "Ruszajmy!", but also "Chodźmy!" not "Idźmy".
My impression is that it generally means "I'm going" or even "I'm coming", technically on foot, but not necessarily so. For example:
At the end of the working day - "Idę do domu" - "I'm going home", which might well involve taking the tram. But once in the tram it's "jadę (tramwajem)".
You call to someone to hurry up as you're about to leave, or tell them that dinner's ready - "Już idę" - "I'm just coming" (like "Ja voy" in Spanish).
If somebody asks how you are getting home and the answer is "I'm walking", then I think Poles would probably say "Idę pieszo" ("I'm going on foot").
Exactly so. The problem here is that the intended sense of "I walk" is ambiguous. The present simple does not simply indicate something one does regularly; a first person narrated story might have "I walk quickly up the stairs and look out the window," which is a continuous, present action.