Item Checklist:>water (obviously)>compass>map>trail food (high carb/sugar content, chocolate melts though, so not the best choice)>toilet paper (you don't need a full roll)>some kind of rain coat/poncho>extra pair of socks>bandage>lamp>lighter/matches (preferably both)>some kind of knife (doesn't need to be big)
When hiking for 5 hours (20 - 25 km or something) in spring or summer I would bring at least 1.5 liters of water per person.
Remember that you need more water when hiking uphill. That's actually where you will need most of your water.
For every 500 meters elevation I would bring another half liter. Of course depends on how long the steep parts are. The steeper the more water you need.
If you are hiking outside of forested areas with no shadows where the sun burns down on you, you will also need more water. Obviously.