In reply to James Malloch:
Not stupid at all! If you are transitioning from bigger bags it seems/feels odd to adjust a smaller bag for a run or even a faster day hike.
Those smaller bags are not suppose to reach the hips, but fastened at the waist. If you are running with the pack, just make sure the rest of the straps (after tightening the waist band) are all well tightened, more so if you were using a bigger pack, to keep it all stable on your back for the run.