If you want to know how poorly people adapt and survive, turn off their power for long enough for their food supply to spoil and/or run out. I think it's safe to say that the majority of us living in first world countries would be absolutely screwed, save for a few who really know how to live off the land and have done so. But those few will also have to learn how to form a community of people who think and act like them, because alone is much harder than in small groups. The stupid and the anti-social would be among the first to perish, whether it by by starvation, exposure or some Darwinesque termination.
But one only has to look at tribal culture in third world countries like Africa and South America. Tribes can thrive and exist without all that we have today, and survive very well. Even some into their 70's and 80's. Humans aren't ill-equipped, we are ill-prepared for the most part, but not all of us. Most of the cities would be in ruins fast though, and I'd venture you'd more likely be killed by someone elses bullet than by an animal or the elements.
Our strength comes in our intelligence and our sense of community. Our herd instinct, if you will.