(wind blowing) (pencil scratching) - [Narrator] Sometimes in making the mountains series, the film crew was more surprised by how mountains affect the people they met.

Filming Mira Rai, the marathon runner, showed more than anything the impact mountain life can have.

It, quickly, becomes clear how well adapted Mira is to the extreme altitude during the walking to the start of the marathon race.

It's a 10-day trek to Everest base camp.

As they climb higher, in every breath there's less oxygen.

By 4,000 meters, there's already a third less than at sea level.

- Walking in this altitude, you take two steps and you, oh, I'm very tired.

Very out of breath.

I've kind of lost track of how many days it's taken us to get this far.

You may notice that Mira is not out of breath.

(laughs) - [Mira] Yeah.

Oh my god.

- [Narrator] Mira has 50% more hemoglobin in her blood, so she feels fine.

She's even got the energy to make her own film.

- Me?

- Ah, yeah.

- I feel very tired.

- Really?

- [Alex] Yeah.

Yeah?

- [Mira] Yes.

- [Alex] Do you like going uphill?

Why?

- [Narrator] Without any roads in these mountains, everything has to be moved by either pack animal or by hand.

The 300 kilos of BBC equipment is carried up by six yaks.

- Here's the rest of our camera gear.

Luckily, we don't have to carry this stuff.

Nimah?

- [Nimah] Yes.

- Do you think it's okay to hold some people once this lot comes through?

- [Nimah] Okay.

- Just hold them there, that would be great.

Because this is getting heavy.

Good.

- [Narrator] But keeping up with Mira on her daily training is tough.

(heavy breathing) Each day, the crew's bodies make more and more red blood cells slowly enabling them to acclimatize.

Mira is always a step ahead.

- [Mira] Oh!

(laughs) - [Narrator] The secret to Mira's sporting success lies in her upbringing.

She was born and bred in the mountains, and her ancestors have lived at high altitude for at least 3,000 years.

Her family home is a village in eastern Nepal.

There is no electricity, or running water, and it's a day's trek to the nearest shop.

It's a life of hard physical labor.

To collect water, the community must climb down to a spring 400 meters down the mountain.

Mira's been doing it since she was just five years old.

(speaking in a foreign language) Living at altitude fine tunes the body making wider arteries, more capillaries, and bigger lungs.

But mountian life was also her biggest obstacle.

Like many women in rural Nepal, she was destined to a life of housework.

(speaking in a foreign language) But her mother dreamed of a better future for her daughter.

(speaking in a foreign language) Aged 14, Mira left home looking for adventure.

She ended up in the capital of Nepal, Kathmandu.

It was here in a forest on the edge of the city that she entered her first ever race.

50 kilometers long.

She'd never run such a distance before in her life.

(speaking in a foreign language) But she didn't just complete the race, she won it.

From that day on, Mira went on to win race after race across Nepal.

There is just one race she has yet to attempt, the Everest marathon.

And the exhausted film crew has finally reached the start to film it.

- [Cameraman] Are you ready, Mira?

- Yes, I am ready.

(laughs) I am ready.

- [Cameraman] Good luck, Mira.

- Ah, yes, thank you.

(gun fires) (crowd cheers) - [Narrator] In just a few years, Mira has gone from a tough life in her remote mountain home to the biggest sports star in Nepal.

Winning every race there is in the country.

(crowd cheering) (soft piano music) Now, she also trains children from other mountain villages.

Helping them to become athletes.

(speaking in a foreign language) Since Mira started, the number of girls signing up to racing equals that of boys for the first time.

For those born in these mountains, there are many obstacles in life, but they are also the perfect training grounds for success.