One evening, when Mo Farah was in the Swiss mountains preparing for Chicago marathon, he came across a pack of dogs glaring at him. And then, as he hesitated, the smallest dog pounced. “It was about 6pm and I was running round the lake in St Moritz,” he says. “I paused as there were these dogs on the trail off a lead and I was thinking the owner was going to stop them. But he let one of them go and then chomp – I had to get injections and there was blood.”

Farah winces at the memory. “I carried on but I was so mad that when I went back to the house I was like ‘a dog’s just bitten me, let’s go find the guy and call the police’,” he says. “But I couldn’t find him.”

A wider metaphor is left dangling. The 35-year-old is the most decorated track distance runner in history, with 10 golds and two silvers at Olympic Games and world championships. But he knows the move to the marathon, like that small Swiss pooch, could yet bite him on the rear.

On Friday, Farah insisted he was “100% sure” he would go to the Tokyo Olympics in 2020 and contend for a medal. He is also confident he is in shape to break the European record of 2hr 5min 48sec after long stints at altitude in St Moritz and Park City, Utah. The race in Chicago offers an instant stress test for those ambitions.

There are seven runners in the field who are faster than Farah but, inevitably, the focus has been on one of them: Galen Rupp, his former training partner at the Nike Oregon Project. The meeting between the pair has been hyped like a heavyweight fight, with the popular running website Letsrun.com even having a countdown clock ticking down to Farah v Rupp on its front page.

You can see why. Rupp was the guy who helped Farah settle in Portland in 2010 when he decided to be coached by Alberto Salazar and then bore him no bitterness when he just pipped him for 10,000m gold at London 2012. Farah, in turn, credited Salazar and Rupp for turning him from a perennial contender to the best in the world in his late 20s.

There has been a lot of water under the bridge since then. In 2015, Salazar was put under investigation by the US Anti-Doping Agency for potential anti-doping rule violations – allegations he strenuously denies. While Rupp stayed loyal to his coach and mentor, Farah left last year.

“We’re more rivals now, for sure,” says Farah, who has put his Portland home, with its six bedrooms and seven baths, up for sale for $1.8m (£1.37m). “I’m back in London, he’s in Portland. I don’t really speak to him but athletics is a small place. We see him at the hotel, when we are having dinner they are having dinner, so we say: ‘Hello, and how are you?’

“But at the same time, I know Galen wants to beat me and I want to beat Galen. He’s on home soil, he’s got a target on his back and under more pressure than I am.”

The Briton has won 21 of their 22 meetings on the track. However, they have never raced over a marathon where Rupp may have the stamina edge. As Farah says: “I think I am in most athletes’ heads. But Galen is a different animal. I know how hard he works. And to be honest with you, because I’m not hiding it, in some sessions he would beat me. On the longer runs he would drop me.”

Since then, however, Farah has broken Steve Jones’ British marathon record – and his time of 2:06:21 was all the more impressive given the record temperatures in London in April. There was also no disgrace in coming third behind the world record holder Eliud Kipchoge. This time round, however, only a win will do.

Rupp, meanwhile, is self‑deprecating when it comes to assessing his chances against Farah, even though he has the greater experience, having taken Olympic marathon bronze in 2016 and won last year’s race here. “I hesitate to call it a rivalry, given my record,” he says, smiling. “He’s been pretty dominant. He got tripped and that was the only reason I beat him so you can put a little asterisk by that one.”

The American also insists they remain friends. “We probably don’t talk as much as when we were living in Portland, but I sent him well wishes after the London marathon,” he says. “He sent me well wishes after I ran Prague. He lives in another part of the world but we still talk and are still friendly on good terms.”

Farah and Rupp point out that there are others here with live chances, that the race is not only about them. The world champion, Kenya’s Geoffrey Kirui, is the favourite, despite his modest personal best of 2:06:07, while two Ethiopians, Mosinet Geremew and Birhanu Legese, have run low 2:04s this year.

Farah believes he will show that his decision to leave Salazar to work with Gary Lough was the right one. “I am grateful for what Alberto has done for me over the years, and there is a lot of respect, but at the same time my life has changed,” he says. “I’ve got a new coach, a new group and I am in great shape.”

Farah’s confidence comes through in every syllable. But Rupp, and the streets of Chicago, will soon put it to the most stringent of tests.