Eden Rising by Andrew Cunningham

Ben and Lila are two ordinary teenagers forced to rise to extraordinary heights when faced with a world that has suddenly and inexplicably died. Dealing with the sorrow of all they have lost, but the love they have found in each other, they set off on an odyssey that will bring them to the limits of human endurance and face to face with the frailty of their very existence. From the extreme violence of many of the surviving humans toward one another, to a world physically falling apart at the seams, Ben and Lila are determined to make it through the devastation in their quest for a place to quietly share their life together. In the process, they have to become as violent as the world around them in order to survive, while struggling to hold onto the humanity that will keep them sane.