Watanabe's home town of Futaba sits next to the Fukushima Daiichi nuclear plant, which was hit by the March 2011 earthquake and massive tsunami that followed, sparking explosions that scattered radioactive debris and forced the town to be evacuated.
Today, the coastal town remains a no-man's land, and Japan's government estimated that an exclusion order barring families from returning could remain in place for almost half of the town for another decade.
The 28-year-old, who slipped out of the town with dreams of making his fortune in Japanese professional cycling, now finds himself an unlikely spokesman for an effort to ensure that evacuees from Japan's nuclear disaster are not forgotten.
"The consequences from the devastation will continue for many more years, and it will be my life work to keep attention on the issue," said Watanabe, who missed the medal platform at the Beijing Olympics but still managed sixth in the team sprint and was feted by his home town all the same.
"I want to help them and bring them some light as an athlete."
When Watanabe was growing up in Futaba, most people either worked at the nuclear power plant or made a living selling things to those who did, like his father, who wove traditional tatami mats.
Watanabe was a gifted runner but he switched to cycling at high school because of the impact of Yuichiro Kamiyama, who won over $30 million on Japan's professional cycling circuit.
Kamiyama dominated the keirin in the 1990s, a race that features a pace bike and a line of riders that build up speed in a tightly choreographed line until an all-out sprint at the end that can reach speeds of 70 kilometres (43 miles) per hour.
Keirin, after lobbying from Japan, became an Olympic sport at the Sydney games in 2000.
But unlike judo, the other Olympic sport with an origin in Japan, competitors from outside Japan have dominated and the country's only medal so far was the bronze won by Kiyofumi Nagai in 2008.
Watanabe, who made his professional debut in keirin at 19, was training in Tokyo for the world championships last March when the 9.0 magnitude earthquake struck off the coast of northern Japan.
He remembers nervous hours watching scenes of the devastation on television before he could reach his sister on the phone and learned that her family and his parents had escaped alive and unharmed.
In the weeks that followed, he struggled to focus on his training. He raced poorly. He no longer enjoyed the sport.
But his family and his friends who had evacuated Futaba urged him to get back in the saddle.
"They were much cheerful than me and helped me to refocus back on what I can do, which is cycling," he said.
Although he is bitter about the loss of his town, he is also equally worried about the risks of people returning too quickly to the contaminated zone around the Fukushima plant.
"They shouldn't send residents back until it is completely safe. It will affect the children who are our hope to the next generation," he said.
In London, Watanabe will compete in the team sprint competition with three riders. Japan has not selected members yet for the sprint or the keirin.