She doubted she could lose a lot of weight, so Ann Anderson set her sights low.

Twenty pounds.

That was her goal when she signed up for Pete Thomas’ “Lose It Fast/ Lose It Forever” program.

“I’d tried in the past,” Anderson says. “I’d lose 10 pounds and gain it — and more — back.”

But that was two years and 60 pounds ago.

Anderson lost that weight and became more active thanks to Thomas, a walking-talking testimony to the ability to lose weight, keep it off and become fitter and healthier in the process.

Thomas, 44, of Ann Arbor, Mich., once tipped the scales at 401 pounds. He began dropping weight after becoming a contestant on the NBC reality show, The Biggest Loser. Three months in, in May 2005, he was voted off the show.

But he took what he’d learned there, plus what he learned on his own, to become the show’s most successful at-home weight-loss contestant.

He lost 83 pounds on The Biggest Loser and another 102 after leaving the show. He has — literally — become half the man he used to be.

Thomas was so excited about what he had accomplished and what he ha d learned that he started teaching others through seminars, boot camps and corporate wellness programs.

Anderson, 46, had tried to lose weight many ways. She knew she was in for something different the first day of Thomas’ 10-week program.

“I’ll never forget it,” she said. “He said, ‘We need to make a decision to commit now or quit.’ That really struck me because that told me this guy is not in it for money or fame. We had paid for the class and he was telling us ‘I’d rather give you your money back because we want serious people here.’

“Then, when you see that life-size poster of him when he was that 400-pound person, and you see what he looks like now … it’s inspiring.”

Anderson ended up losing 27 pounds during the class, and eventually shed 33 more. She has kept it off for about eight months now.

What made the difference for Anderson, a hair stylist who’d been overweight all her adult life?

She says she learned how to eat smarter and exercise more in ways that made it stick, and it became so routine she doesn’t think about it anymore.

“Every week he focused on a specific topic and gave us homework assignments,’’ she recalls. “It wasn’t just coming to class and sitting and listening. He had challenges every week, and he made them fun and interesting.”

For example, one day the class went to a grocery store and participants competed to find the lowest- and highest-calorie food items. “Some of the foods I thought were low-calorie, weren’t,” Anderson says.

Getting active also made a huge difference for Anderson and others. “I remember the first time he told us to run on a treadmill for 15 minutes,” she says. “I thought, ‘Oh my God! He’s crazy.’ ”

But then he talked her through it. “He said, “Run two minutes, walk one minute.’ Well, you sort of feel like you can do anything for two minutes.”

The two minutes turned into 15, and five months later, in October 2010, she ran her first half- marathon. This October, she’ll run her fifth.

“I think about how crazy I thought he was when he told us to run 15 minutes, and now I’ve been running two hours every Monday for the past two years,” she says.