When the dirty job of separating food and compostable materials from trash and recycling was finished at last year’s Big Sur International Marathon, there were about four bags worth of trash to be taken to the landfill. Not bad for an event that featured about 7,000 participants. That amount of waste “could fit in the trunk of a car,” said Kristin Cushman, founder and executive director of The Offset Project, which has been a partner to the marathon since 2009.