About riding here, lifted from a mate's website:
There are several places in Tokyo for cycle training: Oifuto, the Tamagawa river, the Arakawa river and the Imperial Palace.
Oifuto is a cargo loading area for container ships near to Haneda airport. On a weekday the roads there are busy with large trucks but on any Sunday or national holiday it's a mecca for cyclists. You'll find numerous cycling and triathlon teams cycling in pelatons around a 9.5km loop. It's flat except for one bridge. To get there simply head to Shinagawa at from around 08:00 to 09:00 and follow the riders.
The Tamagawa river has a cycling path but it's pretty narrow and sometimes you're better cycling on the road beside the path.
The Arakawa river has a much better cycle path, but you still have barriers to get through at several places. How to get there: Head North along Meiji Doori (the road that goes all the way from Shibuya to Harajuku, Shinjuku and Ikebukuro. About 3km past Ikebukuro station you reach O-ji station. Go under the tracks and immediately there's an intersection. Go straight and eventually the major road will become a minor road. Eventually you reach a T-Junction. Turn right there and you very soon reach the river.
On a Sunday or national holidays you can cycle around the Imperial Palace but the earlier the better since you're sharing the road with car traffic. It's a 5km loop and definitely not flat.
For serious hills, stick your bike in a bike-bag and head out West by train (Chuo-sen) to the mountains. If you go to the mountains with your friends, 1. Take a route map 2. Make sure your mobile phone is charged and 3. Make sure everyone waits at junctions!
I'll be at Oifuto at 08:00 tomorrow for a couple of hours. But dare I say it here, I will be running there too.
Hope this helps,
Just to get a bit of extra info.
Can you take your bike on the train without putting it in a bag? Is there a charge for the bike?
Do they have lockers at the train station for said bag and food/warm clothing etc?