Just enough water
In a marathon, the winners are often the most dehydrated runners. Drinking too much in fact can lead to swelling of the brain or even death.
So you don't need all those bulky hydration packs commonly found in stores. What you need is something like the DolfinPack.
It holds a litre of water and doesn't bounce like most hydration packs do. There are no plastic clips or buckles, unnecessary straps, or pockets.
It's lightweight a form fitting, designed to be unnoticed during action. Also great for water sports and can be fully submerged in water.
This minimalist hydration pack is most of the time all you need for a run or short hike especially when you're fit enough. Carrying too much water also means you're carrying unnecessary weight.