If it's the ActionChallenge event then you do not really need to carry much. The stops are well stocked. A couple of litres of water should be plenty plus a small snack for the earlier stages. Also during the day you will be going through areas where you can buy food and drink in a real emergency. It is very hilly in parts so I would be reluctant to weigh myself down with unnecessary stuff. The only thing I found that endlessly drinking water, especially from plastic containers, becomes quite unpleasant, so a small amount of concentrated squash to add to your water container might be an idea.