The royal trio would have liked to compete but it was ruled out on the grounds that policing the 26-mile course through the capital would be a security nightmare.
However, William, Kate and Harry will be cheering from the sidelines and plan to use the event on April 23 to promote their campaign to get Britain talking about mental health problems and how to overcome them.
Heads Together, the organisation they created combining the work of eight mental health charities, has been made the marathon's charity of the year and will have 500 runners in the race.
More than half of Britain wants Prince William to succeed the Queen
Royal trio outline next phase of mental health campaign
Security fears stopped William, Kate and Harry from running in London Marathon
At the Institute of Contemporary Arts in London today the three royals set out plans to use the marathon to urge Britons to pay more attention to their mental fitness, just as many do to their physical fitness, and seek help if they need it.
We believe that 2017 can mark a tipping point for mental health
William, 34, said: "There are times when, whoever we are, it is hard to cope with challenges – and when that happens being open and honest and asking for help is life-changing.
"Talking to someone else is a positive and confident step to take – but for too long it has been a case of 'Keep Quiet and Carry On'.
As a result, too many people have suffered in silence for too long, and the effects of this can be devastating."
The trio will use the event to promote their campaign about mental health problems
He, Kate and Harry have tried to focus attention on issues such as male suicide rates and mental health problems affecting military veterans and teenagers among others, in the hope that creating a "national conversation" will help sufferers and those who care about them find ways of alleviating the problems.
Last week the Prime Minister, Theresa May, added her voice to those calling for an end to the stigma surrounding mental health problems.
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But critics warned that without a huge injection of cash, doctors' surgeries and other medical services might struggle to cope with a rush of people seeking treatment.
William, however, insisted: "The three of us are really optimistic that things are changing.
"We believe that 2017 can mark a tipping point for mental health – a moment when more and more people no longer feel they have to bear the weight alone for fear of judgment."
Kate, 35, in a £1,559 Erdem dress, recalled inspirational words from a mother she met last week at a centre helping mothers with personality disorders to bond with their children.
She said: "William, Harry and I have been very privileged to witness in the course of our work countless examples of simple conversations that have changed lives, which were the first step on a path to recovery.
"Just last week at the Anna Freud Centre, I heard from one mother how talking to a support worker was – in her words – like medicine. Simply by having someone there to have a conversation with helped her immensely."
'I heard from one mother how talking to a support worker was like medicine' says Kate
William & Kate's love story
Fri, April 29, 2016
To celebrate Prince William and Kate Middleton's fourth wedding anniversary here are their relationship in pictures.
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William and Kate's love story
Harry, 32, said: "If you want to be fit, healthy and set yourself up for success then your mental fitness is absolutely as important as your physical fitness.
"Everyone would get help for a broken leg, so why not seek help for an issue that could hamper you and others around you?"
The royal trio were supported by a number of celebrities, including former England and Manchester United footballer Rio Ferdinand.
Ferdinand, who has talked eloquently about his grief after his wife Rebecca Ellison died of breast cancer in 2015 aged 34, said: "I remember the days when we would never ever discuss this stuff.
"Certainly not in the dressing room. Because we felt that it was a sign of weakness.
"Things are changing. More and more of us now know talking is a source of strength." He said he, William and Harry shared a "common bond" having all experienced loss.
He added: "Speaking to Harry at length more than anyone else, he understands through experience how important it is to speak.
"And he's held a lot in over the years and he'd like to see that change in the mindset of the general public."
'We believe that 2017 can mark a tipping point for mental health', says Prince William
Channel 5 news presenter Sian Williams will be running the marathon after recovering from breast cancer and writing a book on mental health.
She said: "The amount of passion the royals have for this cause is really impressive.
"They have researched it, they have personal experience because of the jobs they do – William taking about his role in Air Ambulance and Harry and his role in the military.
"You get a very keen sense it's something they have been aware of for a very long time and they feel as though they can help get the talking going."
She added: "Obviously they can't run themselves because there would be 26 miles of royal protection officers so I think having us run it for them as part of a team, they'll be there to cheer us on.
"For me and for them there is no greater cause at the moment than addressing mental health and having a parity between mental and physical health."
Common mental health disorders
Wed, November 2, 2016
Common mental health disorders from anxiety and depression to post-traumatic stress disorder and phobias.
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Stress – Feeling under mental or emotional pressure can lead to sleeping problems, a loss of appetite or difficulty concentrating
Musician Professor Green, whose real name is Stephen Manderson, spoke to William about mental health and masculinity.
He said afterwards: "I've always had anxiety.
"Before I knew it was anxiety I used to tell my nan that I had a bellyache. "Later in life my dad took his own life and so did his brother.
"I know a lot of people who have suffered a hell of a lot due to mental health issues so it's something that means a hell of a lot to me.
"We see being sad or being scared as a vulnerability but there's a real strength to be taken from being honest about how you feel.
"People think that being hard is being strong and it's quite often the opposite.
"The image of masculinity is one of being hard but being hard and being strong are two completely different things.
"That's what I want to get across to people."