I just rode a race/event called Attack!299, which goes from Hanno (Saitama) through to Chino (Nagano).
205km and 3,700m of climbing.
There’s coping mechanisms, but not sure they apply to a race (as much of a worry to me as to you, I’m sure).
Anyway, you/we will be fine (the amount of time it takes is a different question, however!).