Oh lordy, on the fly DMing is hard. Like a 4 hour session of improv with dice.
West Marches doesn't have to be totally on the fly for the DM. The players organize and define their agenda prior to the play session, so the DM should have a little time to prepare. The excursion is planned to visit a specific area of the map, so the DM only needs flesh out a bit of the world at a time.