If you are near San Francisco, one of the most memorable detours you can take is up to Point Reyes, the National Seashore just north of the bay. Even the inland trails here have great coastal views, but the best trail for glimpses of the sea is the Coast Trail in the southern section of the park. This can be combined in an extra long loop with the Ridge Trail for a loop (although this less popular trail is more secluded and overgrown). For a shorter run, start at the Palomarin Trailhead in the south, and do an out and back on the coast trail for up to 9 miles one way.
Another fantastic run/hike is to take your time and do the ~5 mile run from Pierce Point Ranch to Tamales Point.