At dinner the other night, Roberto Duran worked up a sweat getting his baked potato just right.

It was a big potato. A football of a potato, and Robert Duran loved it. He loves to eat. Steaks have disappeared from his dinner plate before the count of 10. And now he had this wonderful Grossinger's resort baked potato. cYou could write a poem about this potato.

Roberto Duran sliced open the potato. He mashed it. He put salt in there and mashed it more. He dropped in three patties of butter, four patties, five. They didn't have Grossinger's steaming, mashed, baked potatoes in the streets of Panama City. Duran never had a baked potato until he was the world's lightweight boxing champion. And now he finished work on his potato by covering it with a mountain of sour cream.

"Potato too big," Freddy Brown said to Duran.

For 50 years, Freddy Brown has been the conscience of his fighters. Freddy's wife, Muriel, is here at Grossinger's in the beautiful Catskills. She sleeps in another room from Freddy. She eats at another table. Freddy Brown, 73, is getting ready for a fight. He has been here with Marciano and now he's with Roberto Duran as trainer and tactician, counselor and mother. The potato is too big, he said.

"Too big, Freddy?" Duran said.

"You take mine," Freddy Brown said, shoving his little naked potato across the table.

By June 20, when Duran fights Sugar Ray Leonard for the welterweight championship, Duran will have been in training -- will have traded big potatoes for small -- for 10 weeks. Never has he trained so long for any of the other 70 fights in his 13-year career.

He weighs 151, down from 165, and he will be at the limit of 147 when he goes against Leonard in Montreal. For his first title fight, against Ken Buchanan, Duran trained eight weeks; for a return fight with the only man to beat him, Esteban DeJesus, Duran trained eight weeks. He knocked out Buchanan in the 13th round, DeJesus in the 12th.

Every morning here, Freddy Brown rousts Duran to go run for an hour. Every afternoon, the fighter works out in front of 200 spectators in the ski lodge, going four to eight rounds with an assortment of sparring partners chosen for their resemblance to Leonard either in size or in speed.Done with sparring, Duran works on the heavy bag and speed bag before winding up with a dazzling show of rope skipping in which he does the last 10 skips from a sitting position, the rope a singing blur.

Applause, applause.

Duran smiles, well-pleased.

Whatever happens June 20, it won't be because Roberto Duran is out of shape. At 29, he has abused his body for two decades, torturing off weight to make the 135-pound lightweight limit. But he has won 69 of 70 fights, 55 by knockout, and to see him work in the ring here is to believe that he is ready for Sugar Ray Leonard.

A bit beefy in the upper body; Duran yet is nimble on his feet and astonishingly quick with the hands that have leveled so many men they are called "manos de piedra" -- hands of stone. Those words, in Spanish, are sewn into the side of his right boxing shoe.

It is not good practice to make Roberto Duran angry. When he knocked out Pedro Mendoza in the first round, Mendoza's wife climbed into the ring, screaming and cursing Duran. "A crazy woman," said Bobby Goodman, the publicity man in Duran's camp here. "So Roberto, 'bam.' After 10 mean rounds with Carlos Palomino, Duran did not console the loser with kind words. "Quit, he told the once-great champion. "You don't have it anymore." w

Sugar Ray Leonard has made Roberto Duran angry.

It was chilling scene. At a press conference announcing the fight for which each man could make $5 million, Leonard looked down from the lectern at Duran and said he didn't want just to beat Duran. "I want to kill you," Leonard said. It was said cruelly, as if Leonard were speaking of vermin.

Duran sat at ringside yesterday with friends, Rubin Blades, a Panamanian singer, who translated quesitons and the fighter's answers. Duran was bored.

Bored until someone asked about Leonard killing him.

"If he wants to kill me," Duran said, "he has to stand up and fight."

Duran's dark eyes grew darker.

"How in hell," said this great fighter who knocked out a man and his wife in the same round, "is Leonard going to stand up and fight?"

The eyes burned darkly.

"Because he'll be running, once he feels my punch."

Although Leonard won the World Boxing Council version oft the welterweight title by knocking out Wilfred Benitez in the 15th round eight months ago, he was impressive only sporadically. He missed with punches consistently. Now Leonard says he was not in good shape for that fight.

"Puro cuento," Duran said to that. The literal translation: Pure fable.

The colloquial meaning: bull feathers.

"Baloney," the interpreter said.

Duran is on record as saying that Benitez "fought a chicken fight against Leonard. Me, no chicken." He says the fight likely will not go 15 rounds because he will not let it go 15.

"I doubt it ever gets to 15 rounds," Duran said. "Leonard has only one fight of 15 rounds, and that fight was a different fight, because Benitez did not pressure him the way I'm going to."

Duran is a small Marciano. He brings sledgehammers to work. He swings them at his opponent's body. "If the foundation crumbles, the building will come down," he says. Rentless fury in the ring, Duran, like Marciano, never stops coming forward.

To Duran's Marciano, Sugar Ray Leonard plays Ali. Taller by 2 1/2 inches, more the artist/dancer than the street fighter/pug, Sugar Ray Leonard will bring in a strong jab, a right hand that grows more powerful with every fight and quick feet and reflexes that carry him out of harm's way. He is undefeated in 27 fights.

Roberto Durana couldn't care less.

"I have not paid attention ot his fights." Duran said. All he will say of Leonard, is "He's a good fighter."

Freddy Brown, at Duran's shoulder, said, "Good fighter. He didn't say Leonard was great."

"Leonard is going to fight," Duran said, "an excellent fighter."

"Roberto wanted this fight more than anything in his life." said Goodman, the publicity man. "It's his chance to prove to everybody what a great fighter he is. The United States boxing writers named him fighter of the decade -- in a tie vote with Ali -- but now Roberto sees Sugar Ray getting all the praise. When Roberto gets in the ring in Montreal, he'll be so ready he'll be walking three feet off the ground. I wouldn't want to be Sugar Ray."