Every one of the 42.195 km of an Olympic marathon are grueling, and even though the races often bring hours of pain and suffering, the rewards overshadow all the struggles - usually.

"I'm suffering with the long distance, but I'm still happy to do the challenging sport," said China's veteran marathoner Zhou Chunxiu, who won a bronze medal at the Beijing Games Sunday. "That moment on the podium makes it all worthwhile for long-distance runners."

Zhou, one of China's top Olympic medal hopefuls, suffered through a spate of injuries in her lead-up to the Beijing Games, derailing her meticulous training program for months.

Paula Radcliffe of Britain (1825) stops for a rest in the women's marathon during the athletics competition at the Beijing 2008 Olympic Games, August 17, 2008. [Agencies]

Beijing's gold medal winner, Constantina Tomescu of Romania, has also battled injuries throughout her career, including a bout with tendonitis in her right knee that made her miss the World Championships last year.

Those two are among the lucky ones, having both scored medals in the world's top-level race. Others couldn't even finish the race.

Of the 82 participants in Beijing, 13 dropped out early.

"I hurt my knee a few days ago. I just can't bend it and it got worse and worse," said Magda Lewy from the US, who eyed a top-10 finishing before pulling out halfway with her injury.

Her compatriot Deena Kastor also suffered an early withdrawal due to injury.

"My foot has been sore for the past week. I thought it was just tendons. They get hyper-sensitive leading up to a marathon," said Kastor, the marathon bronze medalist at the 2004 Athens Games. "It was tight most of the way. I felt a pop in my foot and I can't stand on it.

"I didn't expect to be finishing the marathon in a bus."

An injury also forced defending Olympic champion Mizuki Noguchi of Japan to announce an early withdrawal before the race even began, shattering her hopes of back-to-back Olympics titles.

But plenty of racers were able to run through the pain.

Although suffering a still tender stress fracture in her left leg, world record holder Paula Radcliffe of Britain, still tried for her first Olympic medal.

"Of course, I could do with a bit more time, but (I'll) just go in and give it a go. That's a hundred times better than watching on TV," she told the BBC before the Games.

"If it was a big city marathon or even a World Championships I'd say do not push it, make sure I was well prepared. But I don't want to sit there watching it wondering what could have happened."

At the Athens Games four years ago, Radcliffe was forced to drop out of the marathon race with dehydration. The nightmare nearly recurred this year when she stopped for a rest during Sunday's race. She finished the race, in the end, coming in 23rd.

"It was really sore," the 34-year-old said about her leg. "At one point I stopped to stretch it out but it came back.

"It's horrible when you have to drop out. It's almost quicker to keep going. If you stop, you're left out there on your own."

Even if the injury slowed her down this time, she assured it would not stop her.

"I'm still keeping my fingers crossed for 2012," she said.