Weinberger, the youngest man in Friday's 10-kilometre open-water race, kept pace with the veterans and held off the reigning world champion to win a bronze medal. It was Canada's third swimming medal of the London Games.
The 22-year-old from Victoria is the first Canadian to win an Olympic marathon swim medal. Open-water swimming made its Olympic debut four years ago in Beijing, but Canada didn't qualify a swimmer then.
"Training is hard and this is like Christmas morning," Weinberger said. "I just love racing."
Oussama Mellouli — the man Canada's Ryan Cochrane held off for silver in the pool in the 1,500 metres — claimed gold in a time of one hour 49 minutes 55.1 seconds. Weinberger was 5.2 seconds behind the 28-year-old Tunisian.
Mellouli is the first swimmer to win a medal in the pool and in open water at the same Olympics.
Thomas Lurz of Germany and Spyridon Gianniotis of Greece dominated the marathon in recent years. With leader Mellouli pulling away on the last of six laps in Hyde Park's Serpentine, it was a three-man race for silver and bronze between Weinberger, Lurz and Gianniotis.
"At that point it was 'All right, time to get a medal,'" Weinberger said.
Weinberger shook off reigning world champion Gianniotis over the final 300 metres and stayed on the heels of Lurz into the finish area.
Lurz, the bronze medallist in 2008, and Gianniotis are both a decade older than Weinberger. This sport favours race experience and mature engines. Weinberger has neither.
"I'm still working and gaining speed and endurance (to) get my 'old-man strength'" Weinberger said. "That's what I call it."
He and coach Ron Jacks planned for Weinberger to take the pace out quickly. They achieved both as Weinberger stayed away from the flailing arms and legs in the pack, as well as diminished the closing power of Lurz and Gianniotis.
Mellouli, also winner of Olympic gold in the 1,500 in Beijing, was an unknown quantity in open-water swimming. He'd added the marathon to his pool distances just recently.
Weinberger led the pack for two laps and remained among the leaders throughout the race. When Mellouli dropped the hammer on the final lap, Weinberger, Lurz and Gianniotis gave chase and separated themselves from the rest of the field.
The Canadian raced fast, smart and brave to claim a spot on the podium.
"For the most part, what Ron and I came up with is what we planned for," Weinberger said. "Instead of having a race between 25 people, we made the race between four people and just tried to push the pace there and separate things and outsprint the guys at the end."
Weinberger was easily identifiable in the water because he was the only racer in the Serpentine without a cap. He pulled it off early to stay cool in water that was around 21 C.
He'd also removed his cap at a World Cup Olympic qualifier in Portugal a few weeks earlier, only to have to his goggles dislodge. While fiddling with them, Weinberger was mowed over by other swimmers and raced from 49th to second.
"I was smarter this time," he said. "I learned from my mistakes and ripped it off from the front, so my goggles didn't come off."
Weinberger thanked Swimming Canada, Own The Podium, parents Tony and Marina, sister Brittany and his friends back in Victoria who got up early to watch him race. His parents live in Arizona, where Tony is a commercial airline pilot.
Weinberger had the fearlessness of youth on his side. During pre-race introductions, he double-pumped his fist in the air when his name was announced. Weinberger confessed he didn't sleep well the night before and stayed up late watching "silly cat videos."
Jacks says coaching Weinberger is "interesting," which is code for "difficult."
"I'm a basket case," Weinberger has previously said. "I tend to not follow the same idea longer than 24 hours."
But Jacks had nothing but praise for his pupil's concentration Friday.
"Richard, I don't think anybody really realizes what a great race it was today," Jacks said. "He thinks extremely well on his feet in tense situations."
Weinberger made his plan for the future clear when he said "I want to be an Olympic gold medallist in Rio."
He embarked on 100-kilometre training weeks after securing his Olympic spot in June. Wednesday mornings were punishing with five-kilometre swims in quick succession at a hard pace.
"Probably about Wednesday morning, that would be the toughest morning, right Ron?" Weinberger said. "That's when Ron and I would enjoy having lovely conversations with each other."
The Serpentine is flat and cool, which suits Weinberger more than warmer, rougher water.
"We were swimming in seaweed and duck crap," he stated.
The atmosphere in Hyde Park for the men's open-water swim was more relaxed than it was for men's triathlon a few days earlier. With no one from host Britain expected to contend for a medal, there were less media and fewer spectators.
Canada's swim team hit its target of three medals here with Cochrane's silver, Weinberger's bronze and another bronze in 100 freestyle from Brent Hayden of Mission, B.C. The team fell short of getting swimmers into more than 10 finals, with only seven advancing that far.
"The effect of an Olympian and the effect of an Olympic medal is about inspiring the next generation," Swimming Canada chief executive officer Pierre Lafontaine said. "The power of these medals is what they're going to do starting from today on.
"Swimming has to be a lifestyle in Canada. Just in Quebec this year so far over 50 kids have drowned. This should help develop a culture in which every child in Canada should learn to swim.
"If an instructor sees a kid with a little bit of talent, pass him onto the swimming team in the area."
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