IT will be more than a few small steps for astronaut Tim Peake, and a giant leap for the London Marathon on Sunday.

Major Peake, a former Wiltshire resident Army aviator and helicopter test pilot, is busy making last-minute preparations to run the full 26-miles (42kms) while orbiting Earth in the International Space Station.

Floating to the finish line would be cheating, so he will be strapped firmly on to a treadmill while watching the streets of London pass under his feet in real time on an iPad.

In 1999 he finished the London Marathon in a time of three hours, 18 minutes and 50 seconds.

He does not expect to match that performance when he takes part in the Digital Virgin Money event this weekend.

But speaking in a live link to a press conference on Earth, he revealed that he hoped to complete the course in three and a half to four hours.

Major Peake said: "I've been doing longer runs at 7.5mph and I'm confident I can keep that going...

"The fitness training is 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 the astronaut's 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.

Asked about the physical and psychological hurdles ahead, Major Peake said the most testing aspect of his marathon mission had been using the treadmill harness.

The system is designed to mimic the effects of gravity by exerting a downward force on his body and legs.

"The toughest part's been getting used to this harness system," said the astronaut. "It's not very comfortable to run in; it's like running with a clumsy rucksack on. It tugs and pulls in different directions and it causes chafing around the hips and around the shoulders.

"I've kind of worn it in now and got used to it, but that's been quite tough to get used to."

In practice sessions, the harness loading has been set to 75% of his body weight.

Over the last six weeks, Major Peake has been running in the harness for at least an hour every day.

One problem he will not have is getting caught short during the race. Major Peake said the treadmill was conveniently placed next to one of the space station toilets.

"I'm running right next to the loo so there won't be a queue," he joked.

The run will take place in an isolated and windowless part of the space station called Node 3.

But Major Peake said loneliness would not be a problem.

"I'm pretty used to life on board the space station now," he said. "I've been four months up here and I've got my other crew mates who will be popping into Node 3 where I'm running the race. They'll be cheering me on and giving me support.

"I'll have my music to listen to and plenty of things to keep me motivated."

He added: "Unfortunately the treadmill isn't located by the windows. The one thing I won't be able to do is look out the window at planet Earth as I run the race."

Major Peake, who is just over four months into his six-month stint on the ISS, will tackle the course in zero-gravity - on board a treadmill strapped to a large harness.

He will be aiming to finish at around the four-hour mark as he climbs on board the treadmill in the spacecraft's tranquillity node.

Keen runner Major Peake will use elastic straps over his shoulders and around his waist to keep him in contact with the running belt and generate the foot force necessary to give his bones and muscles a workout in weightlessness.

The harness creates a feeling similar to running with a backpack on, and generates enough downforce to stop him from floating away.

And while Major Peake will not have to contend with any rain, wind or other adverse weather conditions that may await the nearly 40,000 competitors on the ground, he will be punished by the harness, which is said to become particularly uncomfortable after around 40 minutes.

He will time his run to coincide with the race start, at 10am, with plans to stream the feat live on the timpeake.esa.int website.