“I fully expected someone to slit my throat but they were going to have to outrun me to do it.” Gilbert Tuhabonye has been running all his life. From genocide and the memory of his classmates being burnt alive, their bodies mutilated by machetes, their skin blistering and dissolving into blackness, through shattering a window with a scalding bone to escape to a mass grave and rebirth. Now he hopes to emerge from the maw of this incredible journey at the Olympic Games.

It is easy to succumb to trite aphorisms when debating the meaning of sport, but at its rawest it provides hope and purpose. Tuhabonye could have remained an embittered victim of man's inhumanity, but he rebuilt his life through athletics. On Sunday, he lines up at the Flora London Marathon, where he will need to shave five minutes off his personal best to qualify for Beijing. “It's a big gap but I can do it,” he said. “I've had a lot of bad luck.”

The bad luck started on October 21, 1993 when the headmaster at his school, Mr Niyonkenguruka, told him that he must see what Jesus saw on the cross. His friend, Severin, ran a finger across his throat. This was the Lycée Kibimba in Burundi and the assassination of the president had opened a Pandora's box of misery.

The uneasy alliance between the Hutus and Tutsis, Tuhabonye's ethnic group, erupted into civil war. “The Hutus lined us up and beat our necks with baseball bats so that we couldn't scream,” he said. “We saw them carrying wood into a building near the school. The women had been taken away. They cut anyone who tried to get away and then, when we were inside, I smelt the gasoline. They threw burning torches inside and people started to die.” Tuhabonye crawled beneath the smouldering bodies of the half-dead. “I watched as the skin on my Tutsi classmates bubbled and blistered and then disappeared.”

Aching with thirst he could not bring himself to drink the pooled rainwater because it contained fatty remains “floating like prisms”. He knew the Hutus would come in to check that more than a hundred Tutsis had perished so he dug deeper beneath the bodies and “felt chunks of seared flesh coming off in my hands”.

“But a voice in my head told me everything would be all right,” Tuhabonye said. “I picked up a femur and swung it like a hammer. The window broke and I heaved myself through and ran.” He ducked beneath a swinging machete and ran to a freshly dug hole in the ground. Only later did he realise it must have been a grave to dump the bodies. The smells were vivid - petrol, burnt flesh, urine, excrement. A Hutu with a torch approached. Tuhabonye knew he would be found and killed. Then the Hutu fell in the hole. “I dug my fingers into his flesh and jerked my arm back with all my strength,” he said. “His neck snapped with a satisfying pop and he crumpled dead.” Finally, he made it to the makeshift triage centre at the Mwaro military camp. Tuhabonye was 18 years old and the sole survivor of the attack on his school.

Tuhabonye had been making a name for himself as an athlete before the attack and, with a shirt and shoes hung over his hospital bed as a reminder of his goals, he defied his burns and mental scars to run. In 1996, the International Olympic Committee set up a camp in LaGrange, Georgia, in the United States, to allow athletes from impoverished nations to take advantage of first-class coaching and facilities. Tuhabonye was selected. He carried the Olympic flame before the Atlanta Games, but did not qualify to compete.

Now 33, he is eyeing the Olympic marathon and although his best time is 2hr 22min 7sec, he says his preparation and tactics have been poor in the past. “I've run 5:05 []pace to 23 miles with no pain, but I didn't take on water and had to walk to the end,” he said. “I've got a fast race in me and the conditions in London will suit me. It would mean everything for me to get to the Olympics. Running has been such a positive force in my life and the Olympics are the pinnacle.”

He would also like to draw attention to the problems that fester in Burundi. It is partly why he decided to write down his story in The Running Man. “People talk a lot about Rwanda and Uganda, but Burundi is often the forgotten country,” he said. “I would love to go back one day but I do not think the situation is safe enough.”

His mind still drifts back to the events of 1993, although he has come to terms with them. “It was hard to deal with having killed a man,” he said. “I struggled to understand how friends could turn against me. I wondered how there could be people like this, how they could kill us and rape the women, but forgiveness is a powerful tool.” He later found out that Severin had been killed in the violence and Niyonkenguruka executed.

Injury curtailed his Olympic hopes in 2004 but this time he believes he has a genuine chance of qualifying. “My strength and endurance are what helped me survive the attack,” he said. Given the running battles of his traumatic past, nobody should bet against Tuhabonye finding the streets of London to be paved with gold.