I'm hoping I make it at least a month - water and medicine will be the most important scarce resource.
I posted this on preban recently. I guess most of it applies here too.
Forget about hunting and fishing off the land for very long - almost all rivers today are stocked directly or indirectly from hatcheries which will die off fast in a true SHTF. And in one season there'll be almost no game left to hunt - no way there's enough natural game to support the tens of thousands of people who'll be out shooting game day and night.
Then there's the bronchitis, pneumonia, typhus, strep, enterogastritis, salmonella, dysentary and staph infections that will spread with no antibiotics or medical treatments.
There's a reason why the average life expectancy before electricity was less than 40.
And then if you're unlucky enough to be stuck in the big cities and do not leave when the SHTF you will really have your work cut out for you. In an urban setting, common street gangs, masses of looters and outright idiots will be the first major opponents you'd have to defend against. CQB/defend-your-base scenarios with sporadic episodes of fierce defensive fighting. Leaving your base of operations to go and "engage" them would be only slightly more suicidal than actually joining them. But after about a week or two they would devour themselves, leaving only the most organized and strongest streetgangs controlling the 'hoods. These large gangs may then begin to disintegrate as food, water and supplies begin to dwindle and they resort to a more urban/tribal existance.
Also, the large roving gangs that already operate on the fringe of society like Hell's Angels or underground "communities" like the Irish Travelers would probably be best suited to survive and move in an organized manner, overtaking smaller, inexperienced pockets of survivors around the countryside. Seems to me these sorts of mobile gang-networks are already fairly self-sufficient, tightknit, probably well armed and probably have plenty of streetsmart-experience needed to survive a full SHTF breakdown of society. They're pretty much a minisociety unto themselves already.
Anyone unfortunate enough to come upon such groups would either be killed outright (no one likes competition) or "owned" into a prison-like existance where your name is 'Nancy' and you're traded around like a pack of cigarettes. Brutal tribalism in the absence of "civilization" is not good. Go to Zimbabwe if you want to see America after a total SHTF breakdown.