What's your rear rack/pannier setup...I'd pair down the hiking gear substantially (i.e. ditch the stove and eat cold food) and strap something like the Ultimate Direction Fastpack 20 to the rear rack. The roll top design should allow you to roll it pretty small. Are there reliable water sources on the hike? then you really don't need to carry much unless conditions are extreme. 15 miles at 3mph will only take 5 hours to cover, you could consider doing it in one long day?