Transit-accessible hiking areas near Los Angeles

get a transit app
for transit directions and arrival times.
Moovit and Transit are good.  Google maps and Apple maps do the job too

get a TAP card
for easy fare payments on most LA area transit systems  [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 and track recording, ideal for dayhiking. optional subscription for more features
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
Poison Oak:  there's a lot of it around here
How dangerous are Bears?  Snakes?  Lions?  (tl;dr: not as bad as you thought)
Hiking 101:  the basics of mileage, elevation and routes
Hiking Glossary:  all the lingo

get more trails
Modern Hiker
HikingGuy
Hikespeak
AllTrails
Hiking Project
LA County Parks Trails

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

get involved
Nature For All:  promoting transit to nature
Trail Angeles:  connecting forest volunteers
Mt Wilson Bicycling Association: maintaining the trails
Lowelifes Respectable Citizens Club: restoring and maintaining trails
Community Nature Connection:  increasing access to the outdoors for LA communities