Out our balcony we can see a little of the road on the east end of our island. Between two apartment buildings and just over the elementary school we usually see cars go by. Today, we're seeing heads. We can't see the bodies underneath, just the heads, bobbing down the road.
I think this map will help you understand what we see once each year. The runners certainly aren't as bunched up as in the photo above, at the start line in Shinjuku. By the time they pass through Harumi, (#19) they've run almost all of the marathon and they just have a few miles left until the finish line at Tokyo Big Sight, which is a huge convention centre.
While I'm not a marathoner, I have run to Tokyo Big Sight (finish line), Shinagawa (#9) Asakusa (#16) and Tokyo Dome (beside #4). I just run those routes on separate days, not all together. I've also run around the Imperial Palace moat, at the center of the map. A local newspaper counted 3,500 runners on that route on a weekday evening, and I've been amazed at how many runners there are there. It's embarrassing to be passed by so many! Last time I ran that course, I was accompanied by a man who visited Canada for the Vancourver Olympics and toured the country coast to coast. We chatted for a few minutes until our paths diverged. It's a great place to run!