Seven marathons, seven continents, in seven days - a monstrous challenge that pilot Mike Allsop was just weeks from undertaking.

But the 42-year-old adventurer received an unexpected blow yesterday.

After training six times a week for up to four hours a day, Allsop found out he had a stress injury to his lower back and would have to postpone his trip.

"I don't give up though," he said.

"I'm going to let it heal, take some advice, have a look at my training, see where I went wrong and look at how I can prevent this happening again."

Allsop, an Air New Zealand Boeing 737 captain from Auckland, was due to start his 7-7-7 challenge on July 15, running his first marathon in the Falkland Islands.

He was then to make his way to Santiago, Los Angeles, London, Casablanca, Hong Kong, and end in Auckland on July 22.

Allsop launched the project in June last year. He has been climbing mountains for 12 years - including Mt Everest in 2007 - and was ready for a new adventure.

He wanted to follow the footsteps of British explorer Sir Ranulph Fiennes, who completed the feat in 2003, and raise funds for the KidsCan charity.

Getting started proved difficult for Allsop, who was never into running. Only a year ago, 6km would end with an "oh my God, what have I done". But he moved on to 10km runs, a half-marathon and eventually a marathon.

"When I ran my first marathon, I was quite scared because it was very, very hard," he said.

"Then I started learning and talking to people and now I enjoy it."

Allsop believed he suffered the back injury in Nepal, where he completed one of the world's highest altitude marathons from the top of the 18,519-foot mountain Kala Patthar last month, as part of his training.

He then carried a heavy backpack for five days when he went with his 7-year-old daughter Maya to see Mt Everest.

He will now have to rest for three weeks, before slowly starting to exercise again. If he heals, the 7-7-7 project will probably launch later this year.

His sponsors Vodafone and Air New Zealand were understanding, he said.

Allsop was still "absolutely, 100 per cent" confident he would complete the challenge.

"My theory is that as long as I'm strong, where my mind goes my body will follow as long as my body doesn't break," he said.

- © Fairfax NZ News