Veterans and serving personnel are competing in a 3,000-mile row across the Atlantic that will see them face 40-foot waves, seasickness and sleep deprivation.
Some of them have been preparing for years, but according to a former Army Major not many know what they are about to face.
"It's a huge challenge, because for however well trained they are, for most of them this is completely a leap into the unknown," says Ian Couch, Lead Safety and Duty Officer for the Talisker Whisky Atlantic Challenge.
"They are going to be at sea for anywhere between 30 and 90 days, so the conditions they are going to get are going to be everything."
Mr Couch has served with the Royal Anglian Regiment for almost 18 years and, most importantly, he is an expert when it comes to safety at sea.
He has previously ridden three oceans and provided safety cover for every crew in the race 2013.
"People get confused and think it's just about rowing, but rowing is the simple part.
"It's having the mental strength and discipline to just face all the small things that grind them down day in and day out."
Mr Couch says that veterans and serving personnel have an advantage on other participants:
"Teamwork is essential. I might be biased being ex-Army, but the military do such a good job at that. They know what it's like to work in a team.
"They have been tired, cold and scared and wet and still just got on and done the job, and that's why they do so well."
He says that it is a "remorseless, relentless and rewarding" experience: "After however many days at sea, when you first see Antigua, you spend the next few hours deciding 'is it actually Antigua or am I just hallucinating again because I am so tired?'
"It's an incredible feeling. The sense of accomplishment, overcoming something which is truly challenging.
"It's the thought of going back and seeing our loved ones, but it is actually tempered often by a slight regret that this amazing all-absorbing experience is coming to an end."