The basemap or background map is not actually offline. the trail lines and the trail text info (meta data) is the only offline currently. The basemap is very large, would take hundreds of MB to GB to store an entire state offline.
However you can pan & zoom around the map while online before heading out on a ride and the Google Maps SDK will cache the tiles.
We hope to someday offer a feature to select an area of the map to download for offline use, maybe in 2018!