Oh, forgot to mention, Griffith Park has many, many miles of trails. Took me a while to hike them all.

And for extra motivation, there are the Sierra Club hikes on Tu, W, Th, all year round. 7-9 in the evening, rain or shine. You learn night hiking, as well as the conditioning. Broken down into 6 levels of effort, so you can go up as you get more fit.

http://angeles.sierraclub.org/griffith/location.asp

Plus, the Sierra Club has much longer dayhikes that happen almost every day. They are led, with a sweep, by very experienced hikers.
I believe the Angeles Chapter has over 3,000 such hikes a year.

And you don't need to be a member to participate in dayhikes.
My favorite of all time was the 15 mile "Lemming Hike"-- from the SF Valley to the ocean.

http://angeles2.sierraclub.org/activities