From the Woods by Charlotte Greene

When Fiona's friends drag her along on a two-week backpacking trip in the remote Colorado mountains, she's pleasantly surprised to learn that she'll be spending that entire trip with their attractive trail guide, Roz.

Once in the woods, however, far from anything resembling civilization, something or someone comes for them.

With their lives in danger, Fiona and Roz are forced to set off on their own to look for help, drawing the threat from the woods and away from their friends but directly toward them.