London Marathon 2015
The start of the elite women's race Peter Cziborra/Reuters

The 26th London Marathon is underway with thousands of participants including celebrities pounding the streets of the capital. Today's 26.2 mile (42km) journey across London will see one of the 38,000 runners become the one millionth person to cross the finish line in the ultimate test of endurance.

The nation's favourite spaceman, Tim Peake, will deliver a good luck message to participants as well as a 10-second countdown. Rather than watch from the best seat in the universe, Peak will undertake an impressive feat which is completely out of this world by running the marathon whilst being strapped to a treadmill on board the International Space Station (ISS).

"I'll eat early, because food doesn't tend to settle well in microgravity," said Major Peake, revealing his winning strategy. "We recover more quickly from sprains and other muscular problems up here.

"We get a video call to our family once a week, so my next one will be at the end of the race," he added. "I won't be able to hug them, but I'll still be looking forward to that throughout the race."

Celebrities taking part include Game of Thrones star Natalie Dormer, double Olympic champion Dame Kelly Holmes (who is running for five charities), new Top Gear host Chris Evans and TV judge Robert Rinder. Former Olympic rower James Cracknell was the fastest celebrity in 2015, completing the race in an impressive 02.50.43. No less than 33 world records were set in last year's event.

The races for elite wheelchair competitors, ambulant athletes and elite women are underway. The Elite Men and the British Athletics plus Men and Women Mass Start gets going at 10.00am BST.

Last year's winners in both the male and female elite categories also make their return. Kenya's Eliud Kipchoge and Ethiopia's Tigist Tufa are hoping to beat their finishing times of 02.04.42 and 02.23.22 respectively.

A record 247,069 applied to take part in the renowned event which weaves through the capital and passes iconic landmarks including Tower Bridge, Big Ben and the Houses of Parliament. You can watch the London Marathon on BBC One now before it switches over to BBC Two at 1.30pm BST.