If Marshawn Lynch returns, he will need a rare season to duplicate what he did in 2014. / Drew McKenzie, Sportspress Northwest

As general manager John Schneider explained to 710 ESPN Seattle Tuesday, the Seahawks are asking a favor of Marshawn Lynch: They need the enigmatic, entertaining running back to decide ASAP whether he intends to play in 2015, so the franchise can plan accordingly. Given Seattle’s salary cap considerations, Lynch’s decision will have a major impact on next season’s roster.

“Obviously we think he’s a helluva player,” Schneider told the station. “We want to have him back. He knows that, and his representatives know that. He knows if he comes back he’s not going to be playing at the same (lesser) number he’s scheduled to make.

“Now, whether or not he wants to play next year, I can’t answer that. I don’t know if he knows at this juncture. He needs to find out where he’s at. We’ve played a lot of football over these last two years, and especially with the way this guy runs the ball, it’s taxing on his body. So he has to reset himself and get in that mind frame and get prepared for another season of this.”

Lynch has one year remaining on a four-year deal that calls for him to receive $7 million ($5 million guaranteed, $2 million roster bonus) next season. Reportedly, the Seahawks are willing to go as high as $10 million for 2015 — good thing Lynch isn’t paid by the sentence — as part of an extension, coming off a season in which he accumulated 1,673 yards from scrimmage, scored 17 touchdowns, and averaged 4.7 yards per carry.

But Lynch, with 2,220 carries (regular season and playoffs), turns 29 April 22, and will have to produce an exceedingly rare season in 2015 to duplicate his 2014 numbers: 1,306 rushing yards, 13 TDs.

Only six backs since the adoption of the 16-game schedule (1978) have run for 1,300+ yards and 10+ TDs in their age-29 seasons (ranked by total yards):

An extension of two or more years means the franchise will take a major gamble that Beast Mode hasn’t run its course, since the six running backs listed above represent less than five percent of all backs who have played an age-29 season.

“We would like to work this out as quickly as we can so we can move forward,” Schneider said. “It may not happen overnight. He (Lynch) is extremely important to what we have here and the decisions we make.”

With the Seahawks expected to have about $23 million available under the cap, they could make good use of Lynch’s potential raise by allocating it elsewhere. Some of it could go to CB Byron Maxwell, who will have plenty of suitors (former defensive coordinator Dan Quinn, now head coach of the Atlanta Falcons, covets Maxwell), or to OG James Carpenter.

But no player is more important to Seattle’s offense than Lynch. How much more can Lynch do, and for how much longer, assuming he still has a hankering to play?

While most backs reach their expiration dates by about age 30, Lynch has given no indication he has reached his. Still, when the end comes for the majority of backs, it does so quickly. Unless Lynch is a freak on the order of the Redskins’ John Riggins, who had his best years at ages 34 and 35, Lynch has a three-year window in which to remain a back who produces elite numbers.

An elite season by a back is 1,200-plus yards and 10 or more touchdowns. In the NFL’s expansion era (since 1960), 37, including Lynch, have recorded at least 2,000 career rushing attempts, Lynch is 37th.

As Schneider suggested, not all attempts are equal. Lynch’s running style, amazing to behold, makes him absorb a tremendous amount of abuse. Lynch’s 2,033 regular-season carries might have been far more debilitating to him than, say, Eric Dickerson’s 2,996 were to him.

Consider that only seven backs produced an elite season at 30 or beyond, with Riggins and Thomas Jones the only ones to do it twice:

Excluding Riggins, the greatest “old” running back ever, all of the above were out of the NFL within an average of three years following their elite seasons. Most were barely productive in two of those years.

Payton had a pair of 1,000-yard years following his final elite season, then was done. Smith had 1,000-yard seasons at ages 31 and 32, but closed his career with three mediocre years. Dillon was a Pro Bowler in 2004 at age 30 and out of the league at 32.

No running back in NFL history went from hero to zero faster than Shaun Alexander, the league’s MVP in 2005 at 28 (1,880 yards, 28 touchdowns) and an ex-player at 31. Injuries had a lot to do with that, but backs become more susceptible as they age, making any kind of long-term extension for Lynch problematic.

As much as the Seahawks need Lynch, it also doesn’t seem likely that Lynch would retire with $10 million, or whatever, on the table, especially with his skills not in obvious decline. Besides, a couple of more years at his current production rate would place him in Hall of Fame conversation — should he care.

Only Lynch knows how his body feels. Lynch is also wired so differently that he might walk away from $10 million. If he does, don’t expect a press conference.