Muir Woods National Monument, run by the National Park Service, is a fantastic opportunity to run among old growth coastal redwoods. There’s also a massive network of other trails in the area, ranging from coastal trails to some steeper, more challenging terrain in adjacent Mount Tamalpais State Park.
The main trails featuring the redwoods are gorgeous, and flat for easy running. The issue is that this area can be very crowded, so it might be difficult — or inappropriate — to run. Go early, or off-season, or in bad weather(!). We’ve also found that if you get 1-2 miles beyond the Visitor Center, things get a lot less crowded and it’s more runnable. This link is an excellent guide to area hiking/walking/running, with details on the trails, distance and elevation. Best to buy a trail map at the Visitor Center.
The easiest running is on the Main Trail, up to 2 miles out and back. But crowded! There are also some nice opportunities to get in some hills on the Canopy View and Fern Creek trails. The Ben Johnson Trail is great, with wonderful variety, but it’s a 925 foot gain.
For a run on a wider trail with good footing, we recommend the Deer Park Fire Road, which parallels the Dipsea Trail. You’ll emerge from the redwood forest and run through exposed grasslands, with great views. It’s a steady, steep climb, rising 1200 feet over 2.4 miles to where it meets the Coastal Fire Road. Up to 4.8 miles out and back, 1,193 foot gain.