Junky,
If you are going to be doing a lot of roadmarch prep, go get yourself some pantyhose at the PX. Cut em off half-way and put em on. They will keep you from chaffing even if you ran the entire 12 miles.
Put plenty of foot powder inside your socks. Dont just rub it on your feet. Actually pour some extra inside for the sweat you'll generate at that high a pace.
Pad ANYTHING that rubs on you. I have heard modern day rucks dont rub like the old Large, external frame, Rucks we used to use in my day. I still have spots on my back rubbed to something "other than" flesh from years of Humping.
Take the tips or leave em. But these are tips that got me through Niemegan in the 80's, Ranger School in the 90's, and ummm, Army Advanced Land Navigation School
in West Virginia.
ReDhAwK