Running is a very popular sport in Japan and the Tokyo Marathon is hugely oversubscribed at registration time (usually in August).
But even if you are not running the event, there’s a festive atmosphere along the course so it’s worth checking out. Good places to watch are near the starting point: the Tokyo Metropolitan Government Building in Shinjuku, near Tokyo Station where the race finishes, or in Ginza where you can combine viewing with some shopping—but wherever you watch it from, expect crowds.
This year, there will be 38,000 runners—37,500 for the marathon and 500 for the 10km race.
Note that although watching the race is free, entering is not—check the official site for participant fees.