Osprey has a new series of running packs, the Rev series (I currently own and love their small 1.5 liter Rev)- they make one in 24 liters http://www.ospreypacks.com/en/product/endurance__trail_fitness/rev_24 which I think might (think and might be the key words!) be possible to accommodate a 3 day, two night run outing.
Gear would include: shelter/sleeping- 40 degree down quilt, short Neoair, event bivy; clothing-light down jacket, rain jacket, windshirt, windpants, beanie, gloves; cooking/food-esbit ti stove, 600 ti cup, ti spork, fuel, lighter, 2 boil in a bag suppers, granola w/ dried fruit, coffee and numerous and asst. bars, blocks, gels, etc; misc: sunscreen, camera, first aid kit, TP, headlamp, compass, map, Micropur tabs
the rain jacket, windshirt and pants would fit on the front w/ the bungee setup (my small Rev I carry my rain jacket and windshirt like that); it comes w/ a 2.5 liter bladder- I'd replace that w/ the 1.5 liter one that comes w/ the 1.5 Rev as my couple of planned runs would be where water is pretty plentiful
much of the food (bars/blocks/gels) and some of the other misc items would be carried in the various stretch pockets on the harness and the small hipbelt has a couple of pockets as well
I've considered going no cook, but after 30-ish miles I'm thinking a warm meal might be just what the doctor ordered :)
It's pushing things a bit, but your thoughts if it seems plausible