Went on a ride today.

I have been inspired by Journey Across Japan, where a guy rode 50km or so a day to tour Japan. And the anime Yowamushi Pedal, which shows an impossible fantasy for me (having friends).

I want to ride across Korea, travel from Busan to Seoul. I intend to do this next year. How far can i realistically travel in a day? There is one huge mountain, but the rest of the route is almost totally flat