Don't get too paranoid. Most people will never encounter such a situation or be dead by the opening credits, running around a city with an M4 is for the privileged few.
Any survival course will tell you that three things interlink to create a survivor: kit, knowledge and mental attitude, and the latter is the most important. There are some very real threats, but to let them weigh too much on one's mind is actually worse for your survival, because the paranoia, stress and insecurity is sure to shorten your life expectancy, whereas unless in the small chance it happens, a smallpox attack on big cities won't.
The first step is to make a threat assessment that realistically takes into account what you might face and identify and pre-plan your actions on a particular situation, even if they run counter to the flee-to-the-hills mentality. Blind panic cuts no ice with vampires.
Having your bug out bag and heading out for some Ray Mears/Andy McNab/McGyver fantasy involving rubbing two boy scouts together to make a fire is fine and all but unless your home is structurally compromised or under direct threat, you're best off staying put - filling the bathtub with water, for example and let's face it, how much food and kit can you carry as opposed to just generally have around the house? And on your way to this survivalist idyll in the backwoods, how much radiation/germs/gridlock/terrorists/zombies, let alone other survivalist loons and worst of all pissed-off, hungry suburbanites (=food?) will you have to face?
All this said, my place is chock a block with outdoors stuff, and having done some wilderness medic stuff I have a fun size trauma pack somewhere under the bed. This is all just by-the-by, not as a concession to impending doom, but I do have a small waterproof go-bag containing a head-torch, cyalumes, paracord, a swiss army knife, duct tape, nitrile gloves, a pocket first aid kit, tuffkut shears, space blanket and kendal mint cake. If I go anywhere with my mountain jacket, there is a whistle and compass tied on and a multitool somewhere as well. Within the last twelve months I've had cause to use everything in it several times apart from the space blanket and mintcake, so it's rather functional and less fancy than it would first seem.
There are plenty of people who are dead because they've worn themselves out lugging around mahoosive survival kits without a clue of how to use them or the good sense to have what's needed to hand, and plenty who are alive because of improvisational ability when caught out with a rusty penknife, some duct-tape and an used elastoplast. But if you want a reasonable off-shelf go-bag, then you'd do worse than look at the Canadian Red Cross's 72h emergency backpack. I believe MEC in Canada sell it as well.