Its supposed to rain tomorrow (I’m in Saitama).
Anyway, my current commute is about half of yours, but I do it on fixed gear bikes, and I have to agree with the others that not having to worry about much, but tire pressure, lubing your chain and tension is great. I recently moved to a clip less set up since I don't wear my "outside" shoes at work, so its perfect and really does make a difference. Also, as many places of employ will pay for your train fare here I essentially make a little money by commuting this way, but I don't know your situation. Scooters and bike salmon are ridiculous here, but u probably know that already. From my experience, Japanese drivers seem to get a little thrown off by people hauling ass with traffic and splitting and/or taking a lane on bikes. I guess this results from the way in which the vast majority of people here see bikes purely as a utilitarian tool for going to the station, shopping etc.
Anyway, you're lucky to have the path along the river, Sumidagawa?