Here's the link to the official site to enter for a chance for a spot.
It's not a fun race. I don't know if any marathons are, but it's a flat course with a lot of community support. Oh, and you get to run through several of Tokyo's most famous areas... not that you'll notice them through the pain.
The weather the last couple of years has not been great, neither has the t-shirt, but it's an interesting experience.