Good question, this is a weird one without any context. The way I understand it is "by the time something happens (referred to earlier) i'm not walking anymore". It can also mean resignation. - Már lekéstük a buszt (we've already missed the bus) - Akkor már nem futok. (I'm not running then.)
Yeah -- that's what I put and was called wrong. At least that makes sense in English!
Does this sentence really mean: "first I was walking, then at some point I stopped walking"? And would "Akkor nem sétálok" without "már" mean "I am not walking then" in the sense that I wasn't even walking before?
That doesn't make sense in English. (That is the second straight translate from Hungarian into English that translated to something that didn't really make sense in English, IMHO)