I started on the first day (in 2008) with the intention to do, I think, 16 miles to Hauser Creek, and carried enough water to dry camp. But I felt good enough that I ended up doing 20 to Lake Morena state park. Made up for it with a couple of easier follow-on days, but in general the availability (or lack thereof) of water sort of encouraged me to start doing bigger days earlier than I had expected.
I think it was good to be in sufficient shape to be able to do this --- if you're confident that you can, you don't need to carry that extra water weight from the start on day 1 (I had sort of the worst of both worlds due to being conservative).
For training I agree with the "just walk a lot" approach, maybe a little cross training in something else if that works for you. For the last couple of weeks before I started I added increasing weight to my pack for training day hikes, but mostly just walked relatively long distances --- a great way to better learn the town I live in was to go on 5 - 15 mile walks around it (without a pack). Of course the lighter your base weight, the less you need to focus on walking with pack on, but I would definitely do most if not all of your walking in the shoes you plan to do the PCT in.
And even then, expect your feet to behave differently in SoCal, be prepared for some blisters --- going through the blister phase is just part of the price of admission, seems to me ...