12 weeks should be plenty of time given your very large strength base. The ruck training is a good idea for your feet, more than anything. I might have stayed in the Corps if all the marching didn't turn my feet into hamburger. Are you a reservist or are you going to Ranger school after basic training? How familiar are you with Army PT? Most of the really tough military schools are mentally demanding, more than anything else. KSC has made a couple of posts on this, so search his comments.