Yup that's a rookie mistake! 90% of guests going to Tokyo Disneyland will get there about an hour before park opening. If you want to get Fastpasses for the most popular attractions, such as Pooh and Monsters, you stand no chance if you're not there then.
Crowds are always huge at weekends, but if you know where you're going and plan effectively, you won't be spending too long in lines. It's not the kind of place you can just turn up and explore though or else you'll be battling crowds all day.
The Japanese are very orderly when it comes to parades. They'll set up blankets etc. and tend to sit down so that people behind them can see. There's no pushing and shoving, or trying to give their kid a better view than everyone else, no kids on shoulder selfishness, it's all very polite and orderly. But you do have to plan around the parades, because once it starts you're pretty much stuck in one place.
Enjoy DisneySea - it shouldn't be quite as crowded, but there's so much to see and it's so big that you'll probably feel a bit overwhelmed. Sindbad is the best ride there, and never has a line, and don't bother even trying for Toy Story, it's the same, but do try your best to do their Tower of Terror.