From the California State Parks website.

This New Year’s Day 2018, California State Parks will participate in the 7th Annual First Day Hikes at parks across the state. This national-led effort encourages people to experience the beautiful natural and cultural resources found in the outdoors with a seasoned guide so that they may be inspired to take advantage of these treasures throughout the year.

Dedicated docents, volunteers and staff will show visitors the wonders of California’s state parks. Distance and rigor will vary per hike/activity, so visitors are encouraged to check out details of start times and description of hikes. For a complete list and detailed information on the California State Park’s First Day Hikes, please visit the First Day Hikes webpage.

Hikes in Marin include Mt. Tam and Angel Island.

Full list of hikes here.