Southern California Trail Running/Camping

Hey all

I'm Canadian, and I'm heading to LA for work starting Monday afternoon. Because it's -3,000,000 degrees here, I thought I would fly out tomorrow, hit up a hotel, then head somewhere to camp for saturday and sunday night. This is a last minute hail mary as I just found out today, but I'm looking for a campsite that has:
1. Low enough elevation so it's not too cold (so sick of winter)
2. In the mountains so I can maybe bag some peaks (but not too high)
3. Access to a couple (or more) trailheads for a saturday and sunday back-to-back (preferably around 10-20 miles per trail)
4. It can be car camping I don't care but I miss sleeping in a tent so I'd like that

Anyway not sure how much traffic this site gets, but I hope one of you can help.

I love each and every one of you.