Long Distance Competition/Ultracycling, Randonneuring and Endurance Cycling - Beginner planning a long distance ride in a month - Please Help!

Well my girlfriend just moved to San Francisco and I am planning to go visit her when summer school ends in September. I was inspired by some youtube videos to make the trip by bike. I am planning on biking every day before then to practice, but I'm going to need lots of advice. And that's why I'm here.

Some major questions I have are:

1) What sort of bike should I buy? I only have a cheap walmart mountain bike to practice on for now.

2) What sort of equipment other than the bike should I bring? Spare tires, tools, food, first aid etc?

3) How long should I expect the trip to take? It is approximately 380 miles from San Bernardino to San Francisco (directly - many more miles by road).

4) Am I allowed to follow major highways on a bike? or only side streets?

5) What sort of training should I be doing aside from lots of riding?

6) What is the most comfortable seat I can get reasonably? I don't mind my legs being tired but I need a good seat.

If you're planning to do the 380 miles all in one go, which is doable ... that's about the same distance as a 600K brevet ... this is the forum for you. I'm slow and I cover a 600K brevet between 34 and 39 hours, depending on weather conditions and terrain ... to give you an idea how long something like that might take.

If you were planning to do it over several days and sleep at night in between those days, the touring forum might be more appropriate:
http://www.bikeforums.net/forumdisplay.php?f=47
Cycletourists often ride approximately 100 km (60 miles) a day, give or take a bit. If that sort of pace appeals to you, your ride might take approx. 6 days.

Thank you for the information, I was planning on taking 6-10 days, yes. I will post this at that other forum then, unless you can move the topic there for me?