Not taking account of how much energy is needed to do long, all-day walks. Took a basic pack-up of jam sarnies, banana, salted crisps and a flapjack but it wasn't enough to sustain me for a full days walking and by late afternoon I was flagging and having to eat a kindly donated Mars bar and some broken bits of Kendal Mint Cake to pick me up and see me to the finish.
Now I take a selection of things to nibble on as we walk to keep the tank topped up (it's no good waiting until you're shivering and shaking, by them your blood sugar is already too low and it takes longer to get yourself back up to speed).