David Sergi vowed to never run another marathon again.

"I swore I wouldn't," said Sergi, 30, of Holbrook. "Then they announced this course, and it was literally too good to pass up. It's worth the torture."

This morning, Sergi, 30, of Holbook, will join thousands of other runners in the inaugural running of the Suffolk half and full marathons. The full marathoners will begin at East Islip's Heckscher Park, weave their way through picturesque downtowns -- Oakdale, Sayville, Bayport-Blue Point and Patchogue -- make a U-turn and end up close to where they started.

As Sergi, who's run five marathons before, describes it, the process can be grueling, sometimes painful, but, in this case, totally worth it.

"It's horrible," he said. "You have to be really dedicated. The course has to be worth it. This course is phenomenal . . . It highlights this community. [It's special] to take 13 miles on this island and say everyone on this 13 miles will commit themselves to this one thing."

Megan Long, a 26-year-old law student at Hofstra, tends to agree. After years of running 5 and 10k races, she's running her first marathon. Her father, she said, has run about 15. Long recently completed her longest run -- a 20-mile jaunt to the Montauk lighthouse and back -- when she was vacationing there from her home in Long Beach.

"I'm nervous and excited," she said as she picked up her race packet at St. Joseph's College in Patchogue Saturday. "I'm nervous about my time . . . [but] it's cool to go through each town. In each town, you're going to have people pushing you and people cheering you on."

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What's more, Long, Sergi and their ilk will be running with a few well-known names. County executive Steve Bellone, who lost 69 pounds and ran the New York City Marathon last year, spearheaded the creation of the Suffolk Marathon and will be running alongside them. A representative from his office said that the legislature would also be running in a relay capacity.

As of Friday, close to 4,000 runners had registered from 22 states, the District of Columbia, and three other countries. The event is also a Boston Marathon qualifier.

Deirdre Donat, who'll be running the half, took on the challenge for the course, too. "It's where I train," she said. She's 63-years-old, has eight grandkids, and took up running eight years ago, when she quit smoking after 45 years. She began running longer distances . . . well, when her daughters made her.

"They did the Disney half," she said of daughters Cara Romano-Sock and Dana Meadows. "And I said, 'Oh, I'm going to watch and cheer you on!' and they said, 'No, you have to run.' . . . I ended up doing the 5k."

Donat, from West Babylon, ran her first half marathon last year, her daughters running with her, and hopes they'll be able to cheer her on. Her other daughter, Jami Romano, already said she'd do her best to be there. Though Jami is working, the proximity of the course to her job makes it possible to take a small break to cheer mom on.

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"Since I live in Suffolk County, I just love this course," Donat said. "This is awesome. You go to a park that you know and I ride my bike there all the time. I think Suffolk County needed to be put on the map and I hope they have one every year."

That type of dedication, excitement and motivation is prevalent among these racers, said Conor Johnson, a race volunteer. A student at St. Joseph's, he wanted to help because he felt like he could be a part of local history, he said.

"The passion of these runners is amazing," said Johnson, 19, of Holbrook. "Just from hearing about all the training . . . I hardly ever run . . . but I think it's a great thing."

It's great enough that Kelly McCormick, Sergi's newly-minted wife, happily put off their honeymoon.

McCormick, a race volunteer, married Sergi seven weeks ago, and he started training the next day. She'll be waiting for him midway to bring a little light to those grueling miles.

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"I'm going to be happy for Mile 13," Sergi said. "She gives me the happy words to keep going."

And disregard that vow about the marathons, he said. If the Suffolk Marathon comes back next year, he planned to be there.

"I say every time, it's my last," he said. "But I'm running New York City."