. . . . Video is too long for DVD. How can I put it on a flash drive? . . . .
If by 'put it on a flash drive' you mean mean burn it using the flash drive as a substitute for the DVD disc, then this is not possible.
Depending on the intended playback device, you can export the video - File, Export movie, and select a format the device supports. However you will have no menus or chapters.
How long is the video?
If more than 2 hrs and less than 4 hrs, you may wish to consider using a Dual Layer (DL) discs if your DVD burner supports them, if you do use DL discs ensure you get good quality ones..