Marin is full of great little trails, some of which weave right down to the ocean. My favorite is the Tennessee Valley Trail, just shy of four miles round trip. Half way out the trail splits for a while, a beautiful but narrow path on the left leading you on a flatter route, and a wider more steep path to the right better suited for bikes and horses. The two paths reconnect before reaching a pebbly cove where I like to sit and watch the waves for a while before heading back. To the right of the beach there is a foot path zigzagging up the hillside that leads to an incredible view of the cove.
I don't get across the Golden Gate Bridge as often as I'd like (hello $6 bridge tolls!) but as I've told you before, Grand View Park, which is just blocks from my house, is a great spot to run stairs and get a 360 view of the city.