Adrian Peterson will celebrate his 29th birthday on Friday.

Wait. Strike that. He's a running back. They don't celebrate birthdays.

Yes, Peterson could be the exception to the rule. Again. He could blow by 30 like a locomotive, break Emmitt Smith's rushing record and play until he's 36, 37, who knows? After all, when you go for 2,097 the season after one of your legs nearly broke off at the knee, well, you have pretty much proved you have a high ceiling and don't deserve sports writers placing any sort of limitations on what you can and can't do.

But it is interesting or, better yet, a sign of the times that Peterson's dependable, primary backup the past four years won't be around as Peterson heads into the stage of his career when common sense suggests he will need a dependable, primary backup most. It's also a sign of the times that Peterson's long-time dependable, backup, Toby Gerhart, left without a peep from any direction to sign a three-year, $10.5 million deal with the Jaguars.

The Vikings went into free agency with a need for depth at running back, but you couldn't have found a single person to rank it among the team's top eight or so needs. General Manager Rick Spielman also made it sound like a low priority.

"I know you can't go into the season with just two running backs," Spielman said. "So that's an area that we've talked about a couple of running backs that are currently out on the market. But we've also honed in on a couple of running backs in the draft as well."

The Vikings aren't doing anything unusual by not rushing to sign a veteran running back to replace Gerhart. Not in a league where the value placed on most veteran running backs lies somewhere south of the value placed on kickers.

There are so many good, young running backs available that it makes little sense to make a big investment in a veteran with NFL wear and tear.

In the Vikings' case, they have Matt Asiata, a 26-year-old with 47 career carries. At 6-feet, 234 pounds, he's not a change-of-pace guy, but he's a powerful runner who shows signs of being the kind of pass protector that Gerhart was (and Peterson isn't) on third downs. Whether Asiata can catch the ball as well as Gerhart remains to be seen, but when the Vikings needed a No. 3 back to step up for the injured Peterson and Gerhart late last season, Asiata proved satisfactory in a pinch.

He carried the ball 30 times for a forehead-splitting 51 yards and three touchdowns in a win over the Eagles. Two weeks later, he ran just 14 times, but gained 115 yards in a season-ending win over the Lions.

The other reasons for the Vikings' patience at running back are two young backs on the practice squad. Both of them are intriguing prospects. Neither of them came into the league with any fanfare, but this league has a long history of backs coming out of nowhere to make an impact.

Joe Banyard, 25, is a 5-11, 210-pounder who saw action in three games for the Vikings last year. He caught one pass for 11 yards and has enough skills to compete for an NFL job.

The other guy on the practice squad is Bradley Randle. Bradley is one of those guys you see in Mankato the first time and say, "Who's that guy?" In Randle's case, I remember saying, "Who's No. 38?" 

You knew he wouldn't make the team, but he was fun to watch. The 5-7, 190-pounder had the kind of moves and speed you'd expect from someone standing 5-7 and wearing an NFL uniform. When he came out of Nevada-Las Vegas last year, he ran a 4.38 40-yard dash.

The Vikings didn't have room for him, so they cut him, signed him to their practice squad, cut him and then re-signed him to their practice squad late last year.

If Asiata can fill the No. 2 role, a guy like Randle could be quite the change of pace from either of the top two guys.