Air New Zealand pilot Mike Allsop is aiming to set a new record for running the world's highest marathon.
The mountaineer and ultra-runner is heading to Nepal this weekend to try to break that record with friend Lisa Tamati.
Starting at 1500km above base camp on Mt Everest at an altitude of between 5300m and 5600m, the pair are aiming to run 42.2km in 10 hours in freezing conditions.
Mr Allsop, who recently ran seven marathons in seven days on seven continents, attempted the marathon last year but ended up with a stress fracture in his spine.
He said in a video about his attempt that he had "some unfinished business".
Ms Tamati, 44, said the Everest world record attempt was the biggest challenge of her year.
She was "nervous and scared, which is probably a good thing".
Temperatures will be about -30C and the pair will also battle oxygen deprivation that will make progress slow and extremely arduous.
"Being an asthmatic makes this a double challenge for my lungs and I am preparing well with preventative inhalers and hoping for a better result than the last two times at altitude at the Manalsu trail race [212km] last November and the year before during La Ultra, a 222km non-stop race over the two highest motorable passes in the world," Ms Tamati, of Taranaki, said in a statement.
The pair will be accompanied to the Himalayas by 11 others and will be guided to base camp by Mr Allsop.
Mike Allsop's greatest climbs
* Mt Everest in Nepal.
* Mt Kilimanjaro in Tanzania, Africa.
* Alpamayo in the Peruvian Andes.
* Every year he runs tours to Everest base camp for Air New Zealand staff.