Today was a whopping 15km. My daughter is out of school until September, and my wife is busy with her job, so I have little time to ride.
I discovered a nice loop which runs around my neighborhood, it is 2.5km long, half of it is flat, then there is a gradual descent, followed by a steep but short climb. I start at Midtown, ride toward Yoyogi Gyoen (a large park), then pass through Aoayama cemetery, then the Tokyo National Art Center, and climb back up to Midtown.
The good thing about this loop is that there are no stops. It's very difficult to ride any distance in central Tokyo without stopping every 100 or 200 meters for a traffic light. I'll probably end up doing laps on this loop until my daughter returns to school, and I have time to do longer rides.