Major Peake, 44, will tie himself to a treadmill on the International Space Station and pound over a virtual start line at the same time as 37,000 other people compete in the London Marathon.
He will also start the event after recording a good-luck message with a 10-second countdown which is to be shown to his fellow runners on a big screen.
Tim finished the London Marathon in a time of three hours, 18 minutes in 1999.
While he does not expect to match that performance today, he is aiming to cross the virtual finish line in around four hours.
He said: “The fitness training has been going well. I’m feeling good. We have a great team that support us up here and they’ve been keeping me in good shape.”
One of his biggest challenges will be having the determination to keep going without the company of spectators or other runners.
To help him, an iPad will screen a moving image of the run which can be adjusted to his pace. In addition, he hopes to have access to a TV showing scenes from the marathon beamed from Earth.
“It will be a huge boost to me to know that I’m running alongside everybody down there,” he said.
Among those competing alongside Tim will be 88-year-old grandmother Iva Barr, who is running the London marathon for the 20th time.
Iva, who first took part in the race in 1982 and plans to keep competing into her 90s, said: “I just love doing the London marathon. It’s like being at the centre of a big street party.
“Training has not been so good this year. I had a fall and hurt my shoulder but one way or another I will get round again.”
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