How hot will it be at night? How much gear are you carrying? I live in a dry climate and I'd normally drink 1.5 - 2L per 100kms in temps up to 27 deg C. So for a 200Km ride I'd only have 4L with me in total. If there is one spot midway to get more water than 2L.
12L seems like an awful lot of water for a night ride.