Imagine running a marathon, but instead of stopping at the finish line, you keep going and run another marathon - and another, and another.

That's the world of Ruth Croft, an ultra-marathoner and one of the world's best trail runners.

Croft came home to the West Coast for Christmas, but a relaxing Kiwi summer was not on her wish list.

She has her sights set on the most competitive trail marathon in the world, the Ultra-Trail du Mont-Blanc. The multi-country marathon involves running 170-kilometres and climbing 10,000 metres.

"It's kind of like the Olympics for trail running," Croft told Seven Sharp.

"It starts in Chamonix, France and you circumnavigate Mont Blanc as you run into Italy, and then into Switzerland, then back into France."

Croft says it's not just a test of physical fitness, with all sorts of mind games also coming into play.

"They say it's experiencing life in a day, like you'll have extreme highs and then it's like really dark lows when it's at mile 130 and everything hurts and you're questioning what you're doing here.

"It's just being, welcoming the 'pain cave'."