How to train to an Hilly Ultra?

I'm planning to run a 60 km with 3400 meter climb in about 3 month.
I'm living in a flat place without any "mountains" routes around (only on weekends I can drive for a more hilly place)
Do you have any suggestions how to train during the week for developing climbing ability?

Thanks....

A treadmill at inclines expected during the race could be great. Especially if it also has a good downhill setting (high end gym treadmills). Or try stadium or office building stairs over and over. But keep it at aerobic effort (conversational) not heavy breathing/speed work. The key is long sustained uphill and downhills.

Forgot to say that training fast downhill is good though to develop muscle impact strength.