Kory Russell, 25, originally from Sully, south Wales, collapsed near the finish line of the Royal Parks Half Marathon in London after suffering a cardiac arrest.
Despite efforts to save him, he was pronounced dead at St Mary’s Hospital in Paddington at 11.55am.
An inquest at Inner West London Coroner’s Court heard that Mr Russell’s body temperature reached more than 40C during the race.
He had no underlying heart conditions and was described as “meticulous” in his preparation for the event, which took place on October 12.
Matt Russell, Kory’s father, said his son had only taken part in short-distance runs and Parkrun events prior to entering the half marathon.
Mr Russell said his son was the eldest of four children and had always enjoyed maths and science.
He had been “proud” to secure a job as a financial officer at State Street Investment Management in Canary Wharf after earning a first-class degree in economics and finance from Loughborough University.
The inquest also heard that Kory had previously worked in a Michelin-starred restaurant in Penarth while studying for his school exams.
Paramedics treated him at the scene and continued CPR for more than 40 minutes—double the 20-minute national guideline.
Assistant coroner Jean Harkin said: “This time was extremely long and shows you just how committed the health services were.”
Dr Langford said: “The priorities of care were ice water cooling to reduce the temperature and chest compression with a Lund University Cardiopulmonary Assist System machine.”
Despite these efforts, Kory’s core temperature could not be reduced and he later died in hospital.
The coroner ruled that he died from cardiac arrest caused by heat exhaustion.
Mrs Russell said: “Kory was the kindest, most loving and hard-working person you could ever meet.
“He gave everything his all, whether it was work, sport, family or friendships.
“He had such a big heart and a personality that would light up any room.
“We are all heartbroken.
“He will be forever missed and remembered with endless love.”
Following her son’s death, Mrs Russell became vice president of Calon Hearts UK, a cardiac screening charity.
City of Cardiff Swim Club said: “The club is deeply saddened to learn of the passing of one of our former swimmers, Kory Russell, who tragically lost his life while taking part in the London Parks Half Marathon in London.
“Kory was just 100 metres from the finish line when he suffered a cardiac arrest and, despite efforts, could not be resuscitated.
“Kory and his sister Seren were both proud City of Cardiff Swimming Club swimmers, always wearing the dragon with honour.
“Kory brought a warm smile to every session, and his politeness, respect, and positive spirit left a lasting impression.
“He was an absolute pleasure to coach and a remarkable young man.
“Our thoughts and heartfelt condolences are with Kory’s family and friends during this incredibly difficult time.
“He will always be remembered as a Cardiff Dragon.”
The Royal Parks Half Marathon organisers also paid tribute.
A spokesman said: “Everyone involved in the organisation of the Royal Parks Half Marathon would like to express sincere condolences to Kory’s family and friends.”
Mr Russell had looked forward to completing his first half marathon and approached the challenge with careful preparation, the inquest heard.
