Why stop at national? This is a global pandemic. (Spoiler: the characters can't know the extent of the pandemic so we can see their shock and dismay when they can't find safety!) This gets back to my criticism that is was just a rip-off / cash-in from TWD's popularity. The end of season 2 (21 episodes) is supposed to take us to the point where Rick awakened from his coma on The Walking Dead. Do we really learn anything that we didn't (eventually) learn from the original series? The community where they were herded to in the 1st season was similar to what happened in Alexandria. The Mexican mansion in season two is Jesse's farm. Despite being conscious for the entire time, the characters will know about what Rick knew after the first season. This is The Walking Dead with different people in a different area of the country.
Pandemics start somewhere. Where did this begin? How is it spread? In 2003, SARS started in China, reached the US in about 40 days and Europe in about 100. Even if we accelerate that model, it would still take months to spread across the globe. I think they wanted to remain focused on this one group in order to not have to explain where, when, and how.
You want them to try and explain how it all went down? Ha ha. That's impossible. The whole premise of the show is silly. Zombies that are dumb as a rock and move slower than my 90 year old grandmother supposedly took over the world. Earth to common sense - there are more bullets than people. The idea that these zombies somehow over powered a machine gun nest or a tank is just ridiculous. But hey, I don't watch it to make sense of it - I just watch it cause it's good drama - TWD anyway. Fear suck hoover vacuum cleaner.
Now if we're talking 28 Days Later or World War Z - that's more realistic.