What percent of people do we figure are still alive in TWD? If there were a 100 people to start and 90 of them became walkers, it would only take 9 days to kill all the walkers if each person could kill 10 a day.
Most people lack that kind of courage, oh, everyone thinks they have it, but the truth is it is much rarer than you admit. Not to mention, even those who have it often lack fighting skills that would ensure their survival. This would work, Hemingway, but I doubt human nature would allow it to happen. In the army, not all guys would fight, some would hide and cower it happens. These are guys trained to do it too. A lot of people would wait for someone else to do something.
Our best weapon would be our intellect, which they seem to have lacked. It has always served us well against animals and lead to us with our tools becoming the apex predator.