Running is a very popular sport in Japan and the Tokyo Marathon is hugely oversubscribed at registration time (usually in August). There were 10.6 applicants for each of the 30,000 places available in the 2015 race.
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.
Note that although watching the race is free, entering is not – check the official site for participant fees.