Transit-accessible hiking areas near Los Angeles

get a transit app
for directions and to know when your bus will arrive.
Transit is a good one.  Moovit is pretty fine, too.

get a TAP card
for easy fare payments with Stored Value.  TAP info

get a map
a paper map and compass, or a GPS app that works without phone service, like these:
Gaia GPS - free offline map ideal for dayhiking. additional maps available with subscription.
AllTrails - great trail guides and info. subscription required for offline use.
Organic Maps - free offline route planning and track recording. open source, no account required.
Hiking Project - free offline map and trail guides.
Komoot - limited free offline map, additional maps available for purchase, no subscription.

get knowledge
Hiking for Beginners:  essential hiking tips
What to Bring on a Day Hike:  water, snack, sun protection, etc
Hiking 101:  the basics of mileage, elevation and routes
Hiking Glossary:  all the lingo
Poison Oak:  stay away from it!
How dangerous are Bears?  Snakes?  Lions?  (tl;dr: not very)

get more trails
Modern Hiker
HikingGuy
Hikespeak
AllTrails
Hiking Project
12 Hikes on Foothill Transit
LA County Parks Trails

get a tent, maybe
and go camping at these transit-accessible campgrounds:
Gould Mesa Trail Camp  (see Pasadena - Arroyo Seco)
Millard Trail Camp  (see Altadena - Millard Canyon)
Mt Lowe Trail Camp  (see Altadena - Echo Mountain)
Musch Trail Camp  (see Topanga - east)
Malibu Creek  (see Agoura Hills - Liberty Canyon south)

get involved
Nature For All:  promoting transit to nature
Community Nature Connection:  increasing access to the outdoors for LA communities
Trail Angeles:  connecting forest volunteers