First, the pack should have both a waist and a chest/sternum strap. Second, it should have enough volume to carry everything you need, with a little additional room for anything extra you might pick up during the day. Third, it helps if it is has some reflective parts for increasing visibility when running during low-light hours. Fourth, if you plan on running in all weather conditions, it should have a raincover. You can buy them separately if the pack does not have one built in.