The comment came out during a recent interview when asked about how much more he thinks he has in the metaphorical tank (around the :40 second mark). He of course thinks he is going to run forever (with the help of the wolverine blood flowing through him) and eventually surpass Smith's career mark.
But can he?
Smith currently holds the record with 18,355 yards; Peterson has 10,315. At his current pace (1445 yards/season and not factoring time lost to injuries) he can overtake Smith sometime during the 2019 season.
That is, if a lot of things work his way. Whatever team he plays with (since he has flirted with leaving Minnesota) will need to have a mammoth offensive line and an offense that features the run (which no one does anymore). He'll have to stave off his team's desire to go to a running back by committee approach (which almost everyone does) as he gets older and slows down a little (its inevitable).
But can he do it?
Absolutely. Great players make great plays and do great things. Yes, there are plenty of things that could happen to keep him from it, but if rushing for 18,000 yards was easy everyone would do it.