Following the River by Sarah Blake

In the distant future, humanity is dying, there are no women left, and all hope is lost, until…
Grace Miller is one of eight women placed in stasis underground before a nuclear blast. But when she comes up to the surface, she is all alone. Now two hundred years into the future, she finds civilization gone, and in its place is a never-ending forest with a mysterious hunter who knows nothing about her world.
She needs to find the other women from her bunker, which means that she has to follow the stranger through the forest. His expectation that she’ll mother future generations of humans infuriates her, but she can’t deny the way her body responds to his.
River is a hunter in the Mountain Tribe. He knows nothing outside of the primitive lifestyle of his people, and even that holds little interest for him. Humanity is dying out, there are only men left, and those who remain are slowly starving.
When he finds Grace stumbling through the forest, he knows two things for certain. The first is that she can save the human race by mating with his tribe. The second is that he wants her all to himself.