We joined the story with Rick in mid-summer, that's not to say the outbreak STARTED then. To go back to my theory again (to tie in the hospital survival), This whole thing starts earlier in spring, when Rick is shot. He's in a coma for a while and it picks up steam. Finally affects his hospital by early summer and is overrun. He's off life support for a few days, week at the most, and wakes up to a destroyed world. Fits the development of the storyline better, puts Shane/Lori into a place that makes more sense, gives time for people like Glenn to have a little experience with walkers so they don't act shocked like Rick is when they see them anymore, etc.
Haven't rewatched the first episode in a while, but in my head, that timeline fits a lot better. Otherwise, there's simply not enough time to get from 0 zombies to ruined world in the time alloted. Not without assigning Rick magically superpowers to survive in the hospital in a coma with no life support for extended periods.