Before Benoit Lecomte waded into the surf off north-east Japan on Tuesday to begin his 5,500-mile swim across the Pacific Ocean to San Francisco, his biggest concern was not the distance, storms, exhaustion or even sharks. The biggest test, he said, will be the monotony.

"The challenge is to keep doing the same thing for hour after hour and day after day", 50-year-old Mr Lecomte told The Telegraph before his departure from Choshi Beach, to the north-east of Tokyo. "I need to be physically prepared, of course, but more than that I need to be mentally ready".

To keep the boredom at bay, and to perhaps stave off thoughts of just how far he still has to go before he passes beneath the Golden Gate Bridge, Mr Lecomte draws up a detailed schedule of what he will think about for each of the eight to 10 hours he will spend in the water during the epic swim.

"I break it down into sections during which I relive past events with my family, or I try to picture a city that I have never visited, walk through its streets and visualise people," he said in. "I try to get into as much detail as possible – to sense the wind or the scent of a person walking by.

"I’m trying to disassociate my mind from by body, and because I’ve been doing this for 20 years now, I’m pretty good at it".

French marathon swimmer Benoit "Ben" Lecomte, takes the start of his attempt of swimming across the Pacific Ocean in ChoshiCredit:

Born near Paris, Mr Lecomte was taught to swim by his father in the rollers of the Atlantic, meaning that he finds lakes and pools far too tame.

He became the first man to swim the Atlantic Ocean without a kickboard in 1998, raising money for cancer research as a tribute to his father. That 3,716-mile journey took 73 days and he was accompanied by a boat that emitted an electromagnetic field to keep sharks at bay. Nevertheless, sonar showed a persistent Great White following Mr Lecomte’s progress for nervous five days.

When he reached the French coast, Mr Lecomte swore he would never try anything similar again. A month later, he had changed his mind.

"Even though the pain was still fresh, I realised that I didn’t want to look back on my life and regret that I had failed to pursue my passion", he said.

French marathon swimmer Benoit "Ben" Lecomte waks down to the water as he begins his attemptCredit:

Mr Lecomte, who was speaking in an interview with The Telegraph three years ago, has been preparing for the swim for six years.

His assault on the Pacific will last up to six months and will follow a similar pattern to his previous long-distance swim. After up to 10 hours in the sea, he will board the support vessel to rest and replace the approximately 8,000 calories he burns while swimming.

The following morning, the support crew uses GPS to return to the spot where Mr Lecomte emerged the previous day and he starts again.

For the first one-third of the journey he hopes to take advantage of the Kuroshio Current, which flows up the east coast of Japan before arcing out into the Pacific. Catching the current could increase his speed to as much as 5 knots.

As well as setting a landmark in human endurance, Mr Lecomte wants the swim to publicise the problem of plastic pollution in the world’s oceans.

"When I started swimming in the ocean, I never used to see plastic", he said. "But now it is everywhere and it is having a serious impact on sea life.

Read more | Plastic pollution

"As a father, I look at my children and I want to do something positive to focus attention on the problem and, hopefully, help to find a solution".

Mr Lecomte’s route will take him through the "Great Pacific garbage patch", a gyre of marine debris in the Northern Pacific that some estimates put at 600,000 square miles.

Mr Lecomte is collaborating with 27 scientific institutions, including NASA, and scientists accompanying his vessel will be carrying out water sampling at more than 1,000 locations for plastic contaminants as well as radiation.

"It is important for me to tell the story of how our personal behaviour is changing the world and that this is the legacy we will pass on to our children", he said. "Anything I can do to reverse that will make the swim worthwhile".