Long before the words had tumbled out of Mo Farah’s mouth, his victory smile gave the game away. After months enduring his own private purgatory, neither bad nor brilliant but somewhere in between, Britain’s double Olympic champion believes he is back near his very best.

Farah was determined to head to Rio with a time of less than 13 minutes in the men’s 5,000 metres in the bank. So when the pace slowed with five laps remaining, he took it on himself to get it. He set off solo, increasing the murderous pace over the final mile before finishing with a 56.95sec last lap to finish in 12min 59.29sec, nearly 15sec clear of Scotland’s Andrew Butchart in second.

It was Farah’s fastest time since June 2012, when he had younger legs and a home Olympics looming into view to inspire him, and afterwards the happiness seeped from him as easily as sweat.

“It’s nice to be able to just be myself and run my own race,” Farah said, beaming after running the fastest 5,000m in the world this year. “It was hard, but it was not a killer. I was satisfied with that one.”

When he was asked whether he had ever been in better shape going into major championships the response was emphatic. “No, never,” he insisted, although whether that was fact or kidology was impossible to discern. Athletes always play their cards particularly close to their chests just before the Olympics. But he was clearly delighted to have found his legs after a mixed year that has included defeats over cross-country, road and track as well another British record in the form of Dave Moorcroft’s old 3,000m best.

“Training is going well,” he said, dedicating his win to his daughter Rihanna, who turned 11 on Friday. “But one thing you can’t forget is it’s harder for me to defend an Olympics. The rest of the guys have had four years of preparation so anything could happen. They haven’t managed it yet but I’m going to have three Ethiopians, three Kenyans, three Ugandans. They’re going to try and do anything to beat me.”

Farah will now head back to Font Romeu, in the French Pyrenees, where he will put the final touches to his training at altitude before flying out to Rio. “I know I need to do another three of four key sessions and that will be it.”

Earlier in the day, the British men’s 4x100m A team of James Dasaolu, Adam Gemili, James Ellington and CJ Ujah took advantage of the lightning fast track in the Olympic Stadium by running a world-best time this year of 37.78sec. But they had to dig deep to hold off the challenge of the British B team, who finished just 0.03sec back.

Ellington said: “I think our running is doing the talking. We are not going to get over confident and start saying we’re going to do this, going to do that. But to leave this track on a high puts us on a wicked path going into the Games.”

Dasaolu, meanwhile, cited the performance of the B team as brightly illuminating Britain’s increased strength in deep in sprinting. “It just bodes well for all eight people who are out there. We’re stronger as a nation. If anyone gets hurt we’ve got people who can step in, we’re like a conveyor belt of sprinting at the moment. That’s our quickest time we’ve run in about 10 years or so, if we run that kind of time in the Olympics we know we’re going to be pushing for a medal.”

Gemili, however, does not believe the team should safely settle for a bronze behind more established sprinting powerhouses. “We believe we are going to push for a gold medal,” he insisted. “We are not there to fill the lane and go behind America and Jamaica. We want to be the best, just like the guys in 2004, it’s possible.”

There was more British success in the men’s 400m as Matthew Hudson-Smith won confidently in 45.03sec, but there was frustration for the European champion Martyn Rooney who was disqualified for a false start. “Apologies to all my family and friends who came to watch me, a false start in a 400m is unacceptable,” Rooney said.

Meanwhile the talented hurdler Andrew Pozzi again showed his enormous potential in his 110m hurdles heat, breaking his personal best by 0.12sec to win in 13.19sec. But again he showed his brittleness as he was forced to pull out of the final, which was won by the Frenchman Dimitri Bascou, with cramp.