20 miles of water varies according to conditions and individual consumption practices.
as far as food for a 38 miler, I'd recommend 250 cal/hr minimum.
a cheaper pack alternative, which both Craig and I use, is the Black Diamond Bbee. not a running pack but works well when running slow, and holds twice as much as the Nathan HPL 20 (which really does not hold much if you use the bladder). this may come in handy if the weather suggests more clothing.
for stomach issues, a combination if Nuun tablets and Tums.

