Nearly November, let's get your members running again. It's cold outside, but show your members the benefits of using apps like Strava, MapMyFitness, RunKeeper, etc to track their movement outside the club, and you'll get all your members more active outside the club as well as filling up those treadmills!
Making this challenge team based could mix things up a bit... just make sure that the instructor who takes the running club out each week is limited on the number of members in their team, otherwise they'll "run away with it".