Welcome. As folk have already said - water!
The first few days are really not too bad and are probably building up in your mind as some sort of monster. Just crack on and be inspired by other peoples diaries on this forum.
It's not like jumping out of a plane where there's no going back once you've leapt into the unknown - if doing Exante feels wrong to you once you've started you can change your mind, but I really don't think you will do as the results will amaze you and you will soon feel great.
Enjoy. XX