Google Maps Bike is my sole source of biking information. Their recommended path is usually pretty good, and then you tweak from there to avoid certain hazards (i.e. double lane freeway entrance, no bike lane, etc.). My path goes from Costa Mesa to Lake Forest, a mere 15 mile trek and that takes about an hour even with trails half of the way.
Only advice to bumping it up from 7 to 30 mi is to take it at a pace, it's not a race. Give yourself plenty of time, it might take around 2 hours+ to make that distance, exacerbated by traffic lights.
Actually... unless you plan to be biking for 4.5-5+ hours every day, which you might physically have trouble keeping up with... you should probably go with driving part ways. Exceptions to this rule join the ranks of the Super Mustachian Elite Cyclers (see sig link and feel free to add yourself).
We're in a drought lately, so rain has not been a problem all summer. And while it might seem cloudy and overcast early mornings, even in the winter days it hardly ever translates in to real rain coming down.
What it really means is that it gets a good bit hotter closer inland, and cools off a lot faster near the coast. Nothing to worry about.
Don't forget to keep your bike clean and salty moisture free.