It took a while — 28 years — for Hilo's BJ Penn to realize that there's a difference between a fighter and a professional fighter.

"It took my whole life to realize that I better start being professional about this whole thing," he said.

The difference is the Penn who has emerged in the last two years. He is now a world champion, a father, a nutrition expert, and considered by some to be the best "pound-for-pound" competitor in mixed martial arts.

His claim to that unofficial title will be at stake on Jan. 31 when he takes on Canada's Georges St-Pierre in one of the most anticipated rematches in the history of the prestigious Ultimate Fighting Championship organization.

Penn vs. St-Pierre will take place at the MGM Grand in Las Vegas, and hundreds of thousands of fans around the world are expected to watch on pay-per-view.

"It's going to be for history," Penn said. "Whoever wins, people will be talking about them as one of the best ever in the sport, and that's a pretty serious thing."

Which is why Penn has become so serious about his lifestyle.

Gone, he said, is the "crazy Hilo boy" who wasn't afraid to fight anyone — even on the street — and then celebrate the victory at a bar or nightclub.

"I have no regrets," said Penn, who will turn 30 in two weeks. "I'm glad how my career went, wins and losses. That's what made me the way I am today."

The scary part is, Penn was on his way to becoming one of the greatest in the sport even with his previous vices.

Penn is considered an ultra-talent in the sport, hence the nickname "The Prodigy."

He was winning jiu-jitsu world championships before he transitioned to MMA, and then took the UFC by storm. He won the UFC welterweight championship in 2004, defeating Matt Hughes for the title.

He left the UFC for two years because of a contract dispute, then returned in 2006 with losses to St-Pierre and Hughes.

He points to the first bout with St-Pierre as an example of the "old BJ."

"I didn't really care," Penn said. "I felt like I was going to walk in and destroy him. I wasn't even taking that fight serious."

Penn lost that bout by split decision, an outcome he realized could have been prevented had he been more fit. "I gassed," he admits.

Still, the outcome was considered somewhat controversial. Penn likes to point out that he went to an after-party following the bout, while St-Pierre needed treatment at a hospital.

"The more I go back to look at it, the more I feel like I won the fight," Penn said. "But there were a lot of Canadian fans and they were yelling so loud, I think it really did (sway) the judges."

St-Pierre has a different view of that first bout.

"The first round, I got cut on the eye, and I was seeing him in double, so I believe I lost that round," he said. "But after that round, my corner helped me wipe my eyes so I could see. Then I came back and won the second and third rounds, so I won two out of the three (rounds)."

Later in 2006 — on his 28th birthday in December — Penn said he awoke to the revelation that his late nights out needed to turn into early mornings in the gym.

"In a way, I'm glad that last fight with Georges went the way it did," Penn said. "Because I'm more focused now."

Now, Penn said he stays away from alcohol and eats healthy. He has all his meals prepared by a personal chef.

And he still trains — and coaches — daily at his gym in Hilo.

"It's unbelievable," said Penn, who will turn 30 on Dec. 13. "I'm just more prepared this time."

At night, he sleeps in an oxygen tent ... when he's not changing diapers for his newborn daughter.

Penn's girlfriend, Shealen Uaiwa, gave birth to Aeva Lili'u Penn last month.

"She's awesome," Penn said, while showing off pictures of his daughter to reporters.

Under his new training regimen, Penn has recorded three impressive victories in the lightweight division, including a choke-out win over Joe Stevenson for the world title in January, and then a knockout victory over former lightweight champ Sean Sherk in May.

The Jan. 31 bout is scheduled for five 5-minute rounds, and Penn said he is training to go the distance, although he would like to finish it earlier.

"Everybody says (St-Pierre) is good at everything," Penn said. "He's not going to be good when I'm on his back, choking him, and sooner or later, it's going to come to that."

St-Pierre, 27, is also known for his strict training methods in Montreal.

Both fighters said they will spend the next two months training with "specialists" in various disciplines, such as wrestling, kickboxing and jiu-jitsu.

"Basically, we all cancel each other out on the training stuff, so it comes down to who's the best guy," Penn said.

In their first meeting two years ago, neither was a world champion (Penn had been stripped of his welterweight title after the contract dispute). This time, both are world champs.

Penn is the UFC lightweight (155 pounds) world title holder and has a professional record of 13-4-1. He is seeking to become the first competitor in UFC history to hold two world championships at the same time.

"I think that would cement him as one of the best of all-time," UFC president Dana White said.

St-Pierre is 17-2, and the UFC welterweight (170) world champion. White said a St-Pierre victory on Jan. 31 could set up a subsequent bout against middleweight (185 pounds) world champ Anderson Silva.

"Right now I think Anderson Silva is the best pound-for-pound fighter in the world," White said. "The winner of BJ Penn and Georges St-Pierre moves right up there. And if Georges wins, he could get a shot at Anderson."

White said last week's UFC heavyweight championship bout between Brock Lesnar and Randy Couture is expected to set pay-per-view records for the organization. He thinks the Jan. 31 card could be just as popular, which would equate to more than 1 million buys.

"I'm just coming off a promotion where I got done saying it's the biggest fight in UFC history in terms of most pay-per-view buys," White said. "I truly believe this fight has the potential to break that. BJ and Georges are heavyweights when it comes to star power."

NOTES

Shane Nelson, who is one of Penn's students, will fight on The Ultimate Fighter Finale show on Dec. 13 at Las Vegas. Nelson is a contestant on this season's Ultimate Fighter series on Spike TV. Match-ups for the finale have yet to be determined.

Brandon Wolff of Kailua will fight on the UFC's "Fight Night for the Troops" card on Dec. 10. Wolff is scheduled to face Ben Saunders in a welterweight bout. It will be Wolff's first fight in the UFC organization.