Apocalypse 030 - book one

Hopefully I can develop this into a full series of books. The thin plot is that aliens that have been dormant since medieval times have been uncovered by miners in Nevada. The aliens' origins were uncovered days before humanity was devastated. They were named 'Mortai'by our ancestors and were driven underground in small numbers but hundreds of years spent underground allowed them to reproduce in recent years. When they were released they spread like wildfire, slaughtering humanity within a matter of months globally. Millions infest the planet, the number slowly reaching a billion. Humanity is struggling to cling onto life, desperate to not become extinct. Our main protagonist, Kat, is a teenage girl and we follow her story of her surviving the apocalypse. In this book she is attempting to reach the safe zone on the outskirts of the city but what will she have to go through to reach her destination..?