I meant to indicate that all of the options you proposed were acceptable.
Sometimes we use the square brackets to show that a word is optional.
So, "what" is indeed optional, and both "going" and "doing" are acceptable, in my opinion, as well as "walking" and "driving." 
I'd say that "going" is the most legitimate, but all are used casually.
You can also say, "I can't go any faster than I already am [going]."