I trail run to work at least a couple times a week, about 3-6 miles each way. This thing is perfect for throwing lunch, phone, and keys in. You really can't fit much, but you don't want to be running with a ton on your back anyway. Just cinch it up higher than you would a normal backpack, so that the waist strap is actually at your waist and not at your belt. That way it really just doesn't flop around at all.
It fits me perfectly, although looks small on me, and I'm 6'2" 195lbs. It seemed a wee bit big on my significant other who is only 5'3" 120lbs, but I feel like it still might work.