There must be fifty good hikes in Marin. I've probably done a third of them, with one of the best being the Dipsea Trail. There's also the race of the same name which is a killer -- 7 miles with about 2300 ft. vertical along the way. One of the nicest of Marin's is up at Point Reyes -- the trail to the top of Mt. Wittenberg. But there are lots of trails closer -- in Southern Marin -- like Tennessee Valley -- off of which several ridgeline trails (Coyote Ridge, for example) are really spectacular. They are good fro hiking or trail running. Anyone needing directions, let me know. I'm up on those trails lots.
I don't usually do ones quite that long, since I'm often running them and I'd just run out of gas. However, going from the Mountain Home Inn trailhead off Panoramic Highway up to the top of Mt. Tam via the railroad grade then the (I think) Fern Creek trail is a nice one. I'll have to check my map but there's also a trail that starts out on the other side of Fairfax that is called the Three Lakes Loop which is good couple of hour hike. I'll do another more detailed posting on Monday.