YOU SAID:

You should build up speed for twelve hours.

INTO JAPANESE

12時間かけてスピードを上げなければなりません。

BACK INTO ENGLISH

You have to get up to speed over 12 hours.

INTO JAPANESE

12時間かけてスピードを上げなければなりません。

BACK INTO ENGLISH

You have to get up to speed over 12 hours.