You should choose a trail-running pack with a large enough water bladder to accommodate your longest run as well as enough pockets to carry food, emergency gear, clothing and other essentials. Most athletes require a 2-litre pack for a 1–2 hour run or a 6-litre pack for a trail run longer than 6 hours. If you aren't looking to bring water on your run, you might consider a lightweight hip pack or waist pack.