All she can see through the churning wash is a dumpy fishing boat, her dizzying, rocking companion for the voyage.

Her legs kick in a constant paddle. Her arms metronomically turn into the 11-degree air and back into the 14-degree water.

Marathon swimming is not a spectator sport, though maybe it should be. At first it's mildly interesting, then vastly dull, then eventually the astonishing achievement becomes apparent, through a kind of Stockholm Syndrome.

Marathon swimming is a great way to turn a pleasant autumn boat cruise into water torture.

Splash, splash, splash, splash. A tick-tock stroke of (as measured by the official observer) 60-61 beats a minute, again and again, unwavering.

There will be 10 hours of this.

Ten hours.

Ten freaking hours.

You have to wonder why.

McCardel is an interesting athlete. Many athletes aren't. Ask them how they did so well and, if they're honest, they'll say they were born with a freakish gift and practised a lot.

But McCardel was born with a good-but-not-great physical gift, and has turned herself into one of the world's best open-water marathon swimmers by sheer force of will.

And that's interesting, because most of us have trouble just making ourselves get out of bed in the morning.

McCardel had a great 2015. She was chosen for the International Marathon Swimming Hall of Fame and became only the fourth person ever to swim the channel three times in a row non-stop (well, technically she's allowed 10 minutes on the beach at each turn-around). It was the first time this has been done in 25 years. It took her 36 hours and 12 minutes.

In 2014 she set a new official record for open-water swimming, the longest continuous, unassisted ocean swim in history: 41.5 hours over 124.4km in the Bahamas.

Some time in the next week, weather permitting, McCardel will set another record: the most channel crossings by an Australian, breaking the record of 19 set by Des Renford in 1980.

"I'm super-excited," McCardel says (she's a very positive person, to put it mildly. It's like an Australian beach decided to be a person).

McCardel was a competitive swimmer as a teenager, but eventually realised she would never make the Olympics.

"I would flog myself at training," she says. It wasn't enough.

But she was still left with "that draw of wanting to push my boundaries, that competitiveness streak and wanting to achieve something in sport", she says. "I wanted to make an impact in the sporting world at an elite level."

She was the youngest of four siblings, always denied a place in games she was told she was too small for. She was stubborn. She insisted on proving herself.

So she tried triathlon, but didn't have that elite quality there, either.

Then she hit on the marathon swim. And it fitted her like a glove.

"Marathon swimming, even though it seems like a long distance and in difficult conditions and that is true, it really is all about where your mind is at," she says. "You won't swim one stroke unless your mind's in the right spot. So if you can conquer your internal dialogue, that self-talk, make sure there is no negative self-talk getting in, then you can literally will yourself through anything."

McCardel, 31, is still climbing her Everest of willpower. Her husband Paul says he's never met anyone with such "total and utter determination … it's fundamental to her".

Says McCardel: "I have willed myself through severe hypothermia to finish swims, through really bad envenomation from box jellyfish, through burns from the sun. Pain really is signals to your brain that you are in pain, and that signal you can override. You can swim through enormous amounts of pain, it's just a matter of 'do you want to swim through that'.

"I feel if you're so committed to a goal that you would nearly die for that goal then you can swim through hell and then reach the other side and that's kind of the direction I've been going."

Endurance sport is not beautiful. It's the opposite. It's wrecking, pummelling, stretching a body into an ambulance-worthy state. It's yelling 'keep going' to nerve endings that scream, entirely reasonably, for an end.

Long-distance swimming, like all marathons, isn't merely a body against a distance. It's the mind at war with its meat-puppet.

"I want to be one of the best if not the best marathon swimmers that's ever been," McCardel says. "So I don't give up.

"I will be like, 'well, I'll probably never get this opportunity again if I give up now, because these opportunities come very rarely because of the cost, the energy, the time. I need to do this today, I can't defer it, I need to work through this pain today, right now.'"

It's not as easy as it sounds, and it doesn't sound easy.

Cold-water training is a must. She must get her body into a state where the blood vessels shrink from the surface, protecting her core from the heat-sapping sea. It takes months to achieve and can be lost in weeks.

But the mental game is the real battle.

McCardel says her ideal state while swimming is "zoned out, not actively thinking about different things, people or ideas or abstract thoughts".

She enters a meditative state, "feeling the stroke", her mind pulsing blank with the rhythm of her breath. In this state time flies by.

But pain, or boredom, inevitably intrude. Her body rebels, her mind goes unco-operative. That Zen place is elusive.

She has strategies for this. Sometimes she'll visualise finishing the swim. She did that on her triple-crossing last year: 36 hours in cold water.

"I dreamt about [the finish] as if it was real. Not as a dream. I was actually there. I could feel the sand between my toes, I could look back and see the boat in the distance that had been escorting me along. I felt the sun on my face, I felt the discomfort of having swum for so long. I was living in that moment."

And then, if she's really, really struggling – as she was on that triple-crossing – she will simply count her strokes. From one to 10,600, concentrating on the basic, slow accumulation, trying to distract from what she knew was encroaching hypothermia.

But beyond the tricks, she acknowledges something ineffable.

"You've got this internal dialogue, your brain is picking up pain signals from different areas. Then you've also got this other voice, in your brain or your heart or wherever you want to think it is, the human spirit going 'no I can keep going'. Sometimes you have this conflicting dialogue in your head."

She was drawn to the channel – the iconic challenge. In her first year she decided she'd do a double continuous crossing. About halfway back she was in a Force 5 storm, in pitch darkness on two-metre waves, having swum more for than three times longer than she'd ever managed before. Her crew pulled her out of the water.

She was more determined than ever to try again. And again, and again.

The English Channel has been swum countless times. The Channel Swimming Association has verified more than 1400 solo crossings since 1875 when Captain Matthew Webb smeared himself in porpoise oil and leapt off Dover's Admiralty Pier.

In 2016 alone there have been 73 successful solo crossings and 40+ relays –  and there's literally a queue – the official pilots are taking bookings up to three years ahead.

Many fail, either due to conditions on the channel (it is a dirty, cold mass, torn by strong tides and stuffed with two busy shipping lanes) or physical or mental limitations. One of McCardel's pilots tells me that one year a burly American arrived, strutting with confidence, set off from the English cliffs and made it about 200m before quitting, complaining he "didn't like the taste of the water".

A few have even died, sinking beneath the waves, victims of hypothermia or heart attacks. But, to be honest, a single crossing for McCardel is now just a "tough training session". Breaking Renford's record is a symbolic moment, a celebration, but she is already planning tougher challenges ahead.

"My husband's like 'can't we just take a holiday?' I'm like 'Oh my god we've got so much to do'.

"I feel like I have an amazing opportunity. I've worked so hard over so many years. I'm on this incredible trajectory where if I keep pushing myself … I want to keep going.

"I've reached all my original goals. I've pushed myself to as much as I thought I could have, now it's about pushing the boundaries of marathon swimming.

"It's about pushing the human spirit. Where does our mind go and what can our body do? Do we really know our potential?

"Maybe we can go further. There's a beautiful loop going on in the world. I've got this great momentum and I hope to keep pushing the sport and the human spirit forward."