Jack Whitehall, Ed Sheeran, television presenter and physicist Professor Brian Cox and The Voice winner Jermain Jackman are among celebrities backing a mass endurance event that ends with a mountain climb.
A core team of 20 adventurers including Richard Branson's son Sam and current women's Wimbledon champion Marion Bartoli will undertake the Virgin Strive Challenge, which sees participants get from London to the 14,691ft (4,478m) Matterhorn in the Alps peak using just human power.
The different legs consist of three back-to-back marathons from London to Dover, rowing across the English Channel, cycling more than 559 miles (900km) to Verbier in Switzerland, before a seven-day hike across the Alps and climbing the Matterhorn.
They will be joined at various stages of the trip by around 350 other participants, including comic Jack, Jamie Oliver, Princess Beatrice, Sir Richard Branson and England's 2003 World Cup-winning rugby union star Lawrence Dallaglio, organisers said.
Jack, Jermain and Sir Richard were at the O2, where the challenge starts on August 7, for the launch. It saw several team members scaling the building.
Afterwards Jack, 25, joked that Sir Richard and the other organisers were "idiots" and his main aim was to stay alive, adding that if he was not feeling fit he might take part on a Segway.
Asked which challenge he planned to do he said: "I'm going to see how my physical state is after Soccer Aid (in June) and decide.
"If it is good and I'm really fit then maybe I will do a cycle, but if I am out of shape it may have to be a walking marathon."
The main group undertaking the mammoth challenge includes Sir Richard, Marion, Devereux Innocent smoothies co-founder Richard Reed and Paralympian Ade Adepitan.
Money raised will go to the Big Change charity, set up by a group of friends including Sam Branson, his sister Holly and Beatrice.
Sam, 28, said it would be used to support projects which help young people develop crucial "life skills" he said were often overlooked in schools.
He said: "I am incredibly excited to have created such a challenging event that will hopefully have such a positive impact on young people across the UK. It has been so rewarding seeing it evolve from an idea with my cousin into a massive and very real physical event that will cross three countries, five different sporting disciplines and contain over 350 participants.
"I am sure that the Virgin Strive Challenge will achieve all we set it out to and I am very looking forward to the adventure ahead... Now I just need to get fit enough!"