MINNEAPOLIS -- Whenever he returns to the NFL, Adrian Peterson will come back on the heels of an offseason that seemed to affirm running backs can still be paid like headline players. The Buffalo Bills gave LeSean McCoy $26.5 million guaranteed this winter. DeMarco Murray got $21 million guaranteed from the Philadelphia Eagles, and Marshawn Lynch -- who turns 29 next month -- signed a renegotiated a deal with the Seattle Seahawks that has $15 million of guarantees.

But whether Peterson plays for the Minnesota Vikings or gets traded to a new team (which, as a league source said on Friday, is Peterson's preference), he'll be going up against a harsh on-field reality. The 2012 NFL MVP, who turned 30 on Saturday, will begin the second act of his career at an age where few running backs have enjoyed the kind of success that flowed so freely to Peterson in his 20s.

Only seven running backs in NFL history have logged more than 1,000 carries in their 30s, and just two -- Walter Payton and John Henry Johnson -- averaged more than 4.01 yards per carry. Emmitt Smith had 5,789 yards in his 30s on his way past Payton in the all-time rushing annals; if Peterson still wants to break Smith's all-time rushing record of 18,355 yards, he'd have to be 41 percent more productive than Smith in his 30s. And even if Peterson were to average 5 yards a carry in his 30s, he'd need 1,633 more carries -- or roughly six seasons of feature-back level work -- to match Smith.

The Vikings' interest in bringing Peterson back is clearly rooted in the belief the running back will be primed for a big season in 2015. If Peterson ran for 2,097 yards on his way back from a torn ACL in 2012, the thinking goes, how fired up will he be after sitting out for a year and seeing his reputation sullied?

There is certainly some precedent for MVP-type production from running backs in their early 30s; Tiki Barber ran for 1,860 yards at age 30, and followed it up with 1,662 yards at 31. Payton posted 1,684 yards when he was 31, and Curtis Martin ran for 1,697 at that age. All told, there have been 10 seasons of 1,400 yards or more by backs in their 30s, and seven of them have come since 2000.

It's after the age of 31 where even the running backs in Peterson's elite stratosphere have declined. From age 31 to 32, Smith saw his average carry drop by two-tenths of a yard; Payton's tumbled by more than six-tenths of a yard at that point. Only two backs -- Payton in 1986 and John Riggins in 1983 -- have even surpassed 1,300 yards at age 32, and Riggins needed 375 carries to run for 1,347. If the Vikings wind up trading Peterson to a team that will restructure his contract, it might be wise for a team to pack most of Peterson's guaranteed money in the first two years of the deal.

For his part, Peterson told ESPN last month he's "ready to shock the world" in 2015, and said the en masse exodus of his sponsors was actually a blessing in disguise. "I have no endorsement deals or anything," he said. "I have a lot of time, just to sit back and work out and relax. I actually like that; I’ve noticed the times when I don’t have any endorsement deals, I'm able to come back and be my best. I'm not flying here, I'm not flying there, I'm not up, stressed out about appearances."

Peterson's tumultuous 29th year gave him both a physical respite and a motivational trigger, and if you're betting on one running back to shatter precedent in his 30s, it wouldn't be foolish to wager on Peterson. But the running back has officially entered a realm where success has been fleeting for many of his predecessors. He has logged 612 fewer carries than Payton did before his 30th birthday, and 870 fewer than Smith did before he hit 30.

That might give him the ability to remain more productive into his mid-30s than those players did. But it's probably a good thing that Peterson is ready for a challenge; he's got a tall task in front of him.