If I'm not mistaken Peterson is less than a few hundred yards away from Smith's pace given what they both accomplished after 6 years. Emmitt Smith made the transition from elite player to compiling average player at age 27. For most of his career he was just a guy who racked up carries.
Can AP do that as well? Why not? The difference is Adrian Peterson is still an impact player and will probably be an impact player into his early 30s like Walter Payton because his skill is beyond that of maybe 2-3 backs in the history of the league.
Looking at the numbers longevity is more of a factor in accumulating rushing yards than actual sustained dominance. So Peterson may not have the mind boggling durability to lead the NFL in career carries by far. But I think he can offset that with being a highly productive, high ypc player.
It's rare to improve as a Running Back deep into your career. Barry Sanders did it. Walter Payton did it. Jim Browns back half was more impressive than his front half.
Peterson looks to be that rare back who gets better with age and mileage.
He will break the record one day in my opinion.