Korea can be very cold in the winter. I've often done long runs with the Seoul marathon club when it's been -15%. And I agree its always a pleasure running in Tokyo. Which is one of the reasons why I want to relocate there.
Sorry about taking my time getting back to you. It looks like you've got the right idea. In simple terms to run a marathon well you need enough fuel in the tank to get you round which comes through doing your long steady runs whereby your you accumulate that heavy legged feeling towards the end, and your marathon pace efforts such as progression runs and those which incorporate long intervals .
I thoroughly recommend Jack Daniels' Running Formula which gives precise training paces according to your ability.An excellent book that you can purchase through Amazon