If you are looking for a good hike in LA, that can be hard to find. Not because there aren't a lot of trails to pick from (there are), but because it can be difficult to find a trail with a decent number of miles on it while not having to drive all the way to Mt. Baldy or up the coast. Sure, we have Griffith Park (and I do include one hike here from Griffith), and you can get as much or as little mileage as you want by combo-ing the trails with each other... but it's still right in the middle of the city, and you are never going to get away from the crowds or the urban view. These hikes provide good distance, seclusion, quiet and the illusion that you are far from the city. All within a 30 minute drive from central LA.
This hike only edges out my second favorite hike by millimeters... simply because it's a little closer. Mt. Lowe also has a lot of options, depending on what you feel like doing. It's packed with awesome Altadena history, too, being the site of a tourist resort location built back at the turn of the century. The remains of a railroad, a funicular, a hotel, an overlook and a "rustic" camping area can be found at various points on the mountain. You can go all the way to the top of Lowe (a moderate, fine for beginners, hike) and back down in roughly 12 miles round trip. If you only want to go to the top of Echo Mountain (where the remains of the resort stand), its only 5.4 miles round trip. If you are looking for a more difficult hike, you can climb up Castle Creek to Inspiration Point and back down for just under 10. This route is steeper, but a great workout. If you go on a weekday, you will run into few people on the lower, Echo Mt. portion of the hike, and almost nobody on the upper portions of the trail that lead to Mt. Lowe Camp. After you read the top of Echo, no matter which choice you make to continue, the city below will quickly fall away and you will be in trees and alongside steep gullies. It's a diverse, fun hike, and also dog-friendly.
Big Santa Anita Canyon Loop
This hike would be my favorite if it weren't for the fact that it's further away (east on the 210 to Santa Anita Ave) and that you have to pay for the Adventure Pass to park.... also that parking is a bitch at Chantry Flats. But once you have dealt with the hassle, this is a truly great hike. It's 10 miles round trip if you take this route... parking and then walking down the paved road (which turns to a path after 1/4 mile) towards Sturtevant Falls. Go past the falls, take a left at the fork that leads to Zion (not the trail to Mt. Wilson) and take the trail all the way back to Chantry Flats along the mountainside through shady groves of Manzanita and Eucalyptus. It's a wonderfully easy trail, flat most of the way, and has the benefit of a beautiful waterfall, idyllic creekside hiking and views of the canyon on the last part of the trail. Another that is dog-friendly.
Malibu Creek / MASH Site
This is not as long as the other hikes, more of a leisurely wandering of the area. Malibu Creek State Park is really great - and free if you park along Mullholland and hike in. About every movie you've ever seen has been filmed in here, not to mention its real claim to fame, which is the MASH tv show set - which still remains on its site. Only a couple of rusted cars are left, but rocks outline where the tents used to be. More than the fun value of seeing the set, it's a really beautiful park that's a joy to hike in... and full of various water features, a lake, a grotto and climbing rocks. It's the furthest away of all the hikes on this list, but on a day with no traffic, you can get there in 30 minutes. Sadly, no dogs allowed.
Arroyo Seco - El Prieto Loop
Also known as the JPL hike, this is the one that most people in the area know about. For good reason, its a very pretty canyon, and there are a variety of ways you can wander the area. Specifically, however, I'm talking about taking Arroyo Seco up to the top where the Oakwilde campground is, then continue on out of the canyon, up the hill to where the trail intersects with Brown Canyon fire road. Take a left, and follow the road to where a trail forks off to the right. This will take you down El Prieto Canyon, which is a pretty little creek that lets out right along the road you came in on. It's roughly 12 or so miles, but an easy one... and one that lets you imagine you are far away from the city. Totally dog-friendly, too.