hi!
I also love in Tokyo and commute regularly from Kamishakujii in Nerima ku to Takadanobaba (11 k) and also to Iidabashi (16k). If you are doing 20 k in Tokyo traffic it will take you probably an hour (av of 20 on a commuter with luggage) .
I think the idea of the folder is OK but 2 things: rush hour train with a folding bike? Hmmm...its a big carry. 2. You will get pissed of with going that distance on such a small bike and wish you had bought a roadie with panniers...
let me know if you want any more advice!
cheers
Ash