It is Britain’s most brutal running race – 268 miles non-stop along the Pennine Way, from Edale in Derbyshire to the Scottish Borders, in January. Jasmin Paris, who is still breastfeeding her 14-month-old daughter Rowan, and was expressing milk along the way, this week beat 136 other competitors – including 125 men – from 15 different countries, to win the Montane Spine Race outright. In the process, on Wednesday, she smashed the men’s course record by more than 12 hours.

Paris, who is a vet working at the University of Edinburgh and studying acute myeloid leukaemia, told the Guardian she had planned to wean her daughter before the race began, but two back-to-back viral infections meant baby Rowan refused to take anything except milk for five days, and so by the time of the race she was still breastfeeding to avoid mastitis.

“I had to express during the race so it didn’t get uncomfortable,” she says. “At the first checkpoint, it slowed me down a lot and I had to hurry afterwards to catch back up.” As the race progressed she produced less milk, so the expressing became quicker. “In the end,” she says, “it was less of a problem than I’d feared.”

To fit her training for the Spine Race in around her job and family life, Paris had to get up at 4am each day, when she would head out for runs in the hills around her home near Edinburgh, while her family were still in bed. She also went on plenty of long hikes with her baby. “My coach told me to get a weight vest to practice running with a backpack,” she says. “But I thought, I have a baby, I’ll take her. It was decent training.”

‘I missed my family most at the start’ … Jasmin Paris with 14-month-old Rowan after winning the Montane Spine Race on Wednesday. Photograph: Yann Besrest-Butler/Montane Spine Race

In all, she was out on the course for 83 hours and 12 minutes, pausing to power nap, eat and express milk for a total of only seven hours along the way. She had to carry everything she needed, and had to navigate using a map and compass, mostly alone and in the dark.

But she was relentless. Previous winner Eugeni Roselló Solé from Spain pushed himself so hard chasing her in second place that he was forced to pull out of the race exhausted with four miles to go. Almost 20 hours after she had finished, only two men had completed the race, with everyone else either still out on the course or retired.

Paris says the hardest part was the first 24 hours. “It’s weird, but I missed my family most then, and I still had so far to go.” She says by the end she was completely absorbed in the race, trying to stay ahead of her rivals, and the thought of seeing her daughter again was spurring her on. She has also spoken of hallucinations because of lack of sleep. “On the final section, I kept seeing animals appearing out of every rock,” she says. “And I kept forgetting what I was doing out there.”

Top British ultra runner and former Spine Race finisher Damian Hall says: “Her performance was extraordinary – one of the great British ultra-running performances.

“What’s especially impressive is that it’s her first time doing a race like this – and she was racing against some previous winners. She showed incredible determination and focus. It’s unusual for someone to do so well when they’re new to this type of racing, which is seriously tiring.”

Olympian Jo Pavey says Paris’s performance was “awesome and inspiring … It must have been amazing for her to cross the finish line and have her little one there for a hug.”

What was that like? Paris laughs. “My daughter’s really sociable,” she says. “So she had made friends with all the officials and people at the finish. When she saw me, she wasn’t really that bothered.”

Since finishing, Paris has been caught up in a whirlwind of attention, with television appearances and Chelsea Clinton tweeting about her victory. “In a way, this bit has been harder than the race,” she says. “At least then I just had one job to do: keep putting one foot in front of the other.”

All she craves now is a good night’s sleep. “And then I have a thesis to write,” she says.