PITTSFORD, N.Y. (AP) Out of shape and preparing to serve a four-game suspension, Buffalo Bills running back Karlos Williams is apologetic when bluntly assessing the fix he's put himself in.

''I'm very disappointed in myself. I put the organization, my teammates, my family in a very bad situation,'' he said, upon reporting for the start of training camp this weekend.

''I only can learn from my mistakes this offseason and get ready to get back in shape, get ready to play football, and only can move forward.''

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There's no point looking back on an offseason that went from bad to worse for Williams, who made a significant contribution to Buffalo's NFL-leading rushing attack as a rookie last season.

''You can't change what happened,'' he said. ''All you can do is work, work, work and get ready to play football.''

He has plenty of time to get into shape. Aside from the suspension, Williams isn't cleared for practice and instead is taking part in conditioning drills while opening camp on the active/non-football illness list.

The trouble began in June, when he raised concerns by showing up overweight for a three-day mandatory minicamp. Williams blamed the added bulk on being empathetic to his fiance' and not wanting her to snack alone during her pregnancy.

Though it was never made clear how overweight Williams was, coach Rex Ryan joked the player might be a candidate for Lap-Band surgery.

No one was laughing a few weeks later, when the NFL announced Williams' four-game suspension for violating the league's substance abuse policy.

The 23-year-old Williams chalked it up to being ''a young kid, and just making bad decisions, not thinking things through.''

His stumbles happened as quickly as his ascension last year.

Williams was considered a project when Buffalo drafted him in the fifth round out of Florida State. At 6-foot-1 and 230 pounds, he had the combination of size and speed to play running back. What he lacked was experience at the position after making the switch from safety during the 2013 season.

Williams immediately dispelled any questions by becoming the second NFL player to score touchdowns in his first six games - and at a time when starter LeSean McCoy was hampered by a hamstring injury.

With a powerful, straight-ahead running style, Williams scored three TDs rushing and another receiving in his first four games before missing three with a concussion.

He then returned to score two more rushing and another receiving in his next two games, and finished with 517 yards rushing, 96 receiving and nine touchdowns (seven rushing and two receiving) in 11 games.

The question now is what role Williams will have once he's eligible to return in Week 5.

General manager Doug Whaley provided no guarantees.

''He's a young guy and he's got to understand that everybody that comes into this building or into camp is competing for playing time and cap space,'' Whaley said. ''He's behind the 8-ball right now. And I think he's more disappointed than we are.''

The Bills have depth at running back, starting with Mike Gillislee, who played a complementary role in closing last season. Buffalo also re-signed Boom Herron and added James Wilder Jr., the son of former Buccaneers fullback James Wilder.

Then there's fifth-round pick Jonathan Williams, who faces potential discipline after being arrested in Arkansas on a misdemeanor charge of driving while intoxicated two weeks ago.

Karlos Williams' first concern is getting back on the field.

''I can control me, and all I can control right now is being in shape,'' Williams said. ''It's all about showing people, showing my family, showing my friends, the ownership, my coaches, my teammates and definitely earning my teammates' trust back.''

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