Growing up in the south of Sudan, life was a permanent struggle from one day to the next when the only thing that mattered was survival.
But tomorrow, in what is surely one of the great inspirational stories from London 2012, the 28-year-old United States-based athlete – still officially stateless having not yet received a passport – will be hoping to complete the men's marathon as an independent runner.
Competing under the International Olympic Committee (IOC) flag, Marial's tale is an uplifting account of how he made the transition from refugee to Olympian. If the Games is all about the taking part, look no further than this humble man who is just grateful for the opportunity he has been given.
South Sudan seceded from Sudan last July following a peace deal in 2005 that ended the civil war in which some 1.5 million people died. But it has no National Olympic Committee or, for that matter, running water or electricity in many of its towns and villages.
Indeed, Marial hasn't seen his parents since 1993 but there will be no prouder athlete in tomorrow's race.
"When I was growing up, running meant you were running from danger," said Marial.
"It was the survival of the fittest. I loved football but had no idea what running shoes were. So in 2002 when my teacher said I should start, it took a couple of months to convince me. In Sudan, I used to run to escape with my life."
By the time he was 10, eight of his brothers and sisters had been murdered. He spent most of his youth in refugee camps and had no idea what life was like on the outside.
After several attempts at fleeing, Marial finally managed to get across the border into Egypt. He arrived in America as a 16-year-old and now lives in Arizona where running means everything to him. "Nowadays, when I get up in the morning if you ask me not to run on a particular day I'd say no."
It is no surprise that when invited to compete at London 2012 for Sudan, Marial refused. He could not face representing a country that made his life hell and which is now South Sudan's uncomfortable neighbour.
Getting him into the Olympics as an independent athlete was anything but easy. "It took scores of phone calls," recalled his friend and manager, British-born lawyer Brad Poore. "It kind of snowballed but everyone came together to make it happen."
Marial qualified to run in the Olympics in his first ever marathon last year after starting out as a cross-country runner. His best time is 2 hours 12min 55sec – but his goal is not necessarily to complete the course.
"What I want is to raise awareness for refugees across the world," said Marial, who learned only a week before London 2012 that he could compete. "I'm so fortunate to be here and grateful to the IOC and to Brad whom I consider a brother for taking up my case. I'm running for all refugees just like I was. I can't put into words what this means to me."
Whether or not his parents can get to watch him on television some 3,500 miles away is doubtful since the nearest city is 30 miles distant and walking the only form of transport. "I hope they can but it's long way," said Marial. "It's the rainy season at the moment so there won't be any vehicles to take them."
IOC spokesman Mark Adams summed up what it means for the entire Olympic Movement to have Marial in the race.
"As we say over here, I'm frankly gobsmacked," he said. "For us this is an incredible story – we have had independent athletes running before but this is an incredibly humbling story because this guy is so unique."
Andrew Warshaw is a former sports editor of The European, the newspaper that broke the Bosman story in the 1990s, the most significant issue to shape professional football as we know it today. Before that, he worked for the Associated Press for 13 years in Geneva and London. He is now the chief football reporter for insidethegames and insideworldfootball. Follow him on Twitter.