ssongs, I direct you to
Gaijin Chronicles This is a blog by a black american who has been living in Japan for something like the past 7 years now, initially as an english teacher, but has now settled into the country and even married a japenese girl. In addition to being really funny, it's also extremely informative, telling about Japan from an American perspective.
Here's how I can answer your questions based on his writings. As a foriegner, the people would rather sit closer to a pile of vomit on the train than next to you, but they're also too culturally polite to tell you as such.
They like their girls young there, so the older a woman is there who is still single, the easier it is for you to be able to get into a relationship with them. This is also a country where older than 25 is considered old.
But really I think the culture shock will be so much that hunting for a ladyfriend will be one of the last things on your mind for the first few months. Just read as much of that blog's archives as you can.
