A Hastings woman has retired on the home stretch of the world's longest horse race in Mongolia.

Krista Donnelly, 25, put her paediatric nursing on hold to compete in the 8th Mongol Derby's 1000km course.

The derby recreates the course of Genghis Khan's empire-busting horse messenger system.

Ms Donnelly is one of 41 riders selected to compete in the race, which even features in the Guinness Book of Records.

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She has ridden horses for 17 years and has produced horses to compete in several disciplines.

She has educated young horses on the hunt field and ridden breakers as well as carried out track work for the past six years.

Ms Donnelly is experienced in orienteering at a competitive level and has enjoyed a lot of tramping and camping in remote New Zealand locations.

She said she loved travelling to Middle Eastern countries as a child, so it was no surprise she got involved with the derby.

Looking for a serious challenge which combined horses and travel, Ms Donnelly said this race, which would push her to the limit was "absolute top of [her] bucket list".

She said she had followed the derby in previous years, but now, "I'm ready".

But on the home stretch she had to retire. Of the 41 riders who set out, 14 have retired before the finish line due to injury and general tiredness. Some had even been hospitalised but were now well enough to cheer from the sidelines.

The 20 men and 21 women from 13 different countries set off on the derby on August 4.

There was pre-race training from August 1 to August 3 and the final riders are expected to finish by August 14.

The nurse is one of two New Zealanders who were selected for the derby. Gareth Jones, 45, who is a New Zealander but lives in Ireland, is also competing.

Riders change horse every 40km and either live with herders or camp under the stars.

Krista Donnelly and a riding companion set off at 7am in misty conditions in Mongolia. Photo / Richard Dunwoody
Krista Donnelly and a riding companion set off at 7am in misty conditions in Mongolia. Photo / Richard Dunwoody

Some of the riders are professional or semi-professional, others are enthusiastic amateur riders. They are all vying for the derby crown.

Competitors need to balance survival skills and horsemanship, they must endure the elements, semi-wild horses and unfamiliar food and terrain.

Anti-rainforest destruction charity Cool Earth is the race's official charity, but riders can also raise money for their chosen charity.

Ms Donnelly is raising money for Cure Kids.

The entry fee for the 2016 Mongol Derby is close to $20,000, competitors are also required to raise more than $1000 for both Cool Earth and their chosen charity.

Additionally, the cost of her kit and other supplies for the race took the total cost to $30,000.

The itinerary:

* The 2016 Mongol Derby will run until Sunday.

* Sunday: final riders expected to finish.

* Monday: Riders back to Ulaanbaatar.