Challenge: Miles Frost running a gruelling race in Kenya in 2013 but he died suddenly on Sunday while jogging near Oxfordshire

The bereft family of Sir David Frost today paid tribute to his 'super fit' eldest son who collapsed and died at the age of 31 while out jogging.

Financier Miles Frost fell ill during a run at the family's holiday home in Oxfordshire on Sunday.

It is another devastating blow for his family after Sir David died of a heart attack aged 74 two years ago, and Miles had become 'leader of our pack' since his father's death, loved-ones said today.

Miles was described as a fitness fanatic and marathon runner who had shown so signs of ill health before he died.

Mother Lady Carina Fitzalan-Howard told the Evening Standard: 'Miles quite simply was born an old soul. And everybody was drawn to his wisdom, love, loyalty and laughter.'

He was extremely close to his two younger siblings, and they had lived together in recent years and all three boys went to Eton.

Drinks entrepreneur George, 28, and TV presenter Wilfred, 29, said: 'Miles was the kindest and most generous brother we could have wished for.

'Since Dad died he was the leader of our pack. Everyone who Miles met immediately fell in love with his warmth, selflessness and incredible sense of humour. He has gone far too soon and is so undeserving of this.'

Shocked friends said the keen athlete had appeared fit and healthy and was in 'great form' in the last few weeks and showed no signs of any health problems.

Mr Frost, the Eton-educated eldest son of TV legend Sir David and his wife Lady Carina, was a founding partner of private equity group Frost Brooks.

Lord Chadlington, a family friend of the Frosts, said: 'Miles was a wonderful eldest son who was very well loved by all his friends and family. This is a terrible shock.

'I had breakfast with him last Thursday and he was in great form. There was no signs that anything was wrong.

'It just underscores the fragility of life. There will be a post mortem examination but all we know is that he collapsed while jogging.

'The Frost family are extremely close and they will all miss him terribly, as we all will. It is hard to lose a loved one at any age, but 31 really is difficult to comprehend.'

Much loved: Lady Carina and the late Sir David Frost with their son Miles in 2008, far right, and siblings Wilfred, far left, and George, centre

Mr Frost was also said to have socialised with friends on Friday without any indications of ill health and appeared at a charity cricket match two weeks ago.

One friend said: 'This appears to have come out of nowhere. Miles was in great form recently and seemed really fit and well. It's a terrible shock and everybody feels desperately sorry for his mother and brothers.

'He was a wonderful man who has gone far too early.'

Mr Frost had previously run a half marathon in the Lewa Game Reserve in Kenya, regarded as one of the hardest long distance challenges because of the heat and high altitude.

Prince William, an ambassador for the Tusk charity which works towards protecting wildlife in Kenya, was said to have been hoping to run the 2013 race with him. But he was unable to take part because the Duchess of Cambridge was due to give birth to Prince George around that time.

Before founding Frost Brooks, Mr Frost spent five years at LDC, a top private equity firm, where he completed several investments in the technology, media and telecoms sector.

Mr Frost helped lead prayers at last year's memorial service for his father (pictured together in 2012), who shot to fame in the early 1960s

On his company website he described how his personal network includes 'some of the most respected names in business and media', many of which had invested with Frost Brooks.

Earlier this year, he made headlines after his firm invested £3million in BizEquity, a leading US group that provides company owners with an online valuation of their business.

Mr Frost helped lead prayers at last year's memorial service for his father, who shot to fame in the early 1960s in his weekly satirical revue That Was The Week That Was.

Sir David went on to interview many of the world's leaders, famously including disgraced US president Richard Nixon, in which he finally teased some belated admissions of guilt over the Watergate scandal.

His career spanned journalism, comedy writing and daytime television presenting, including The Frost Report. He had been due to interview Prime Minister David Cameron in the week that he died.

Sir David was travelling alone in August 2013 when he had a heart attack on board the Cunard liner, where he was due to give a speech.

After first launching his career in the UK in the mid 1960s, Frost traveled across the Atlantic and found fame in the United States in the late 1960s and early 1970s with the David Frost Show.

During his time fronting his eponymous program he interviewed luminaries such as Muhammad Ali and the playwright Tennessee Williams.

However, the erudite broadcaster indisputably scored his biggest success as a broadcaster in 1977, when he finally got former president Richard Nixon to agree to a sit down talk about his involvement in the Watergate Scandal that forced him from the White House in 1974.

Close family: Miles Frost, far right, with parents Sir David and Lady Carina and younger brothers Wilfred and George in 2008

Historic: David Frost interviewing former President Richard Nixon in a rented house near Nixon's California property in San Clemente, California. Sir David launched himself back into the public eye in the United States after interviewing President Nixon in 1977. The politician was forced to admit that he had taken part in the infamous Watergate scandal

Duel: His interview with the doomed American President 'Tricky Dicky' Richard Nixon in 1977 was a TV classic

Up until that point Nixon had remained silent since he was pardoned by incoming president Gerald Ford - a decision which split the nation and robbed his detractors of a trial.

Allegedly, Nixon underestimated his British opponent and Frost and Nixon sparred through the first part of the interview, but Frost later said he realized he didn't have what he wanted as it wound down.

Nixon had acknowledged mistakes, but Sir David pressed him on whether that was enough.

Americans, he said, wanted to hear him own up to wrongdoing and acknowledge abuse of power - and 'unless you say it, you're going to be haunted for the rest of your life.'

'That was totally off-the-cuff,' Sir David later said. 'That was totally ad-lib. In fact, I threw my clipboard down just to indicate that it was not prepared in any way ... I just knew at that moment that Richard Nixon was more vulnerable than he'd ever be in his life. And I knew I had to get it right.'

An under pressure Nixon blurted out: 'When the President does it, that means it's not illegal.'

Admitting he was part of the cover up, Nixon eventually conceded that he had let the American public down.

His dramatic interview with Richard Nixon was at the time the most widely watched news interview in the history of TV.

It was shown in almost every televised nation in the world, and garnered the largest audience ever achieved for such an interview in the United States.