When I lived in Pasadena, I boarded at a couple of different places. San Pascual Stables is one of the most utterly convenient situations if it fits for you now:
http://www.sanpascualstables.net
There are trails along the Arroyo Seco, not the most extensive network but very fun to ride. I never felt unsafe though these trails are well used by not only horses but people, people with dogs, etc, so if you don’t like to share, you might find it annoying. The management has changed since I was there. I sure loved being 10 minutes from the barn, and the Arroyo area is very peaceful and lovely year round.
Other places I would suggest are Paddock Riding Club in the Los Feliz area: http://www.thepaddockla.net … they have access into the Griffith Park trail network.
LAEC itself is attached to the huge trail network of Griffith Park. It is not always the most satisfying for horse care, but I have a friend who has been there a while who has worked out a situation that makes her horse happy.
There are many barns in the Shadow Hills and Hansen Dam area. These will let you ride out to the dam area (it is a flood control dam, and there’s not really a lake, so you ride in the lake bed) or up Little Tujunga Canyon. The main Hansen Dam stable usually has room, and they have nice big 12x24 covered runs that can make for nice horsekeeping.
I rode out of this facility for a while, and it’s one that has drylots with multiple horses in it that might be a good fit for you: http://shadowhills-equestriancenter.com
Someone has already suggested the Horsetrader as a good source of info.
If your horse will tolerate being with other horses, and you want to trail ride, you might be able to find a small private situation that fits you well. Shadow Hills has quite a few properties that look like suburban subdivision that have little horse setups in the back. People like this with a slot would advertise in the Horsetrader usually.
The thing to know is that horsekeeping is expensive in the area and you want to compare barns on a complete services-you-need basis. That is, board often doesn’t include turnout, feeding grain, blanketing, or any storage for tack, trailers, or feed, and you may need to pay extra for those services. Some barns feed hay and some barns will only do so at extra charge or if you provide it and pay for space to store it. The larger barns have trainers as tenants so the barn management and the training/handling services are separate - you may find you need to affiliate with a trainer at some of them. Ask about hours too - barns that are located in the city limits often are obligated to be closed or at least dark at night.
All of the places I’ve listed would be within 30 minutes of most places in Pasadena proper (at least when I lived there, lol). If you get to the east end of Pasadena, the commute to these areas, which are all west, will get much worse.
Pasadena is a good place to live in an apartment, but house prices can get expensive fast. We bought a house in the south Glendale area, which was also a very good commute to all these places.