When you about to undertake an usual amount of physical effort ie a trek a charity walk of cycle race where endurance is important it is vital to get your preparation right. This is assuming that you normally are as so many of us are a sedentary worker (sitting down).
On the day of the event you must abandon your cautious attitude to calories, you need to eat a lot, you need to eat regularly, you need to eat BEFORE you get hungry, you need to eat healthily with a balance of slow and fast burning foods. For fast burners eat bananas and quality energy bars.
Similarly you need to drink regularly, the best way is from a platypus style flat pack water bottle which you carry in your backpack with a long tube to drink from.
If you hydrate too little you will suffer energy loss and worse and find it difficult to recover even if plenty of drink is subsequently available.
Platypus Big Zip SL Hydration System – 34835 (USA)
Platypus Hoser Hydration System (UK)
Oh and don’t forget the chafing cream!