Even before Friday's news that he would be listed as doubtful for the final game of the Minnesota Vikings' 2013 season, this year would have to qualify as a trying one for Adrian Peterson. The reigning NFL MVP began with talk of breaking Eric Dickerson's season rushing record after falling just 8 yards short last year, and his team had aspirations of a deep playoff run after a surprising 10-6 finish in 2012.

Only time will tell whether Peterson's 2013 season was an injury-induced fluke, a regression to the mean or the toll of a heavy workload catching up with him. And it's not like he's had a bad season. Assuming he doesn't play Sunday, Peterson will still, in all likelihood, finish among the league's top five or six rushers, despite carrying just 18 times in the Vikings' final four games. He had a shot at his second straight rushing title despite playing much of the year with nagging hamstring, groin and foot injuries that sapped his breakaway speed and made Peterson, by his own admission, run tentatively at times.

But time won't give Peterson -- or the Vikings -- this year back, and by the next time he steps onto a football field, he'll be 29 years old, gearing up for the first of two seasons outdoors. The Vikings' next indoor home game will come when Peterson is 31 years old in 2016. In light of all that, it's not hard to feel a lingering sense that Peterson's time as an elite running back might be slipping away.

History certainly doesn't play in his favor. Peterson has run for at least 1,300 yards in four of his seven NFL seasons; since the 1970 AFL-NFL merger, only 11 running backs have run for at least 1,300 in a season. The number of 1,300-yard seasons by a 30-year-old running back drops to nine, and by age 31, it's down to five. Only one running back since the merger -- Walter Payton in 1986 -- has run for more than 1,300 yards at age 32, and Payton is the only one even to eclipse 1,200 yards at that age.