Probably the best hiking site for the Los Angeles area is This Hiking Trail. It's the site where I first discovered my favorite hike. I just recently discovered the trailhead to a hike very close to where I live. It's a moderate distance at 7.8 miles, but is fairly strenuous at over 2000 feet in elevation from start to highest point. The point of all this is to show the usefulness of the site. It lists several dozen area hikes, links to other hiking sites, and (actually useful) comments from other hikers.