Tokyo is one of the more difficult World Majors because you can't carry personal water. You'll have water stations at every 2km or 3km (alternating) - so there is water on the course but often not enough. If you get cramps or are worried about dehydration in the last few kilometers of the race, then this is not the race for you. They're also very strict about the cut-offs and they're not consistent, so be mindful of that.
I had a great experience and enjoyed running the streets of Tokyo.