A day pack that you can actually run with? The Peak 20 fits well and is stable for moderate running, but is also large enough and comfortable enough for light overnights.
Since moving fast on the trails is a lot more fun when your pack isn't bouncing around, we built the Peak 30 with a MotionFit Trail and Airvent Agility+ system. It moves comfortably with you while holding the load stable.
If we could give you a booster pack to get you up the hill, we would. This is the next best thing; perforated shoulder straps, lightweight waist belt, and simple compression keep this bag's weight to a minimum.