Like many of the 1,700 swimmers who will attempt to navigate Auckland's harbour crossing tomorrow morning, Bronwyn Impson's battle will be as much mental as physical.
"I'm terrified. Absolutely terrified," the 34-year South African-born social worker said.
The 2.9km distance should not be too much of a problem for Ms Impson, who is using the event as a training exercise in the build-up to the Taupo half ironman. But to complete the course - which starts at Bayswater's Quinton Park and finishes at Karanga Plaza in the Viaduct Harbour - she'll have to conquer her fear of the open water.
"Swimming has always been one of those things that I can't do," she said. "I got stuck on a different level of evolution."
Ms Impson's journey to the start line began with a marriage break-up four years ago. She asked two friends who had endured divorces how to get through it.
"One said go to therapy, which I did. The other said 'run'."
So she did, negotiating several half-marathons. It helped her find herself, but all that beating of feet took its toll. A rash of injuries made it clear she needed to change. Another friend suggested triathlon.
"I thought, 'Yeah, I can do triathlons. I can run, I can cycle. But, oh crap, I can't swim."'
Changing that has been a "very humbling process", involving technical advice from coach Ben Eitelberg of the Fitness Locker and inspiration from her job working with troubled families in Manurewa.
"Every day I work with people who have been through huge amounts of trauma and are going through things that are really hard for them.
"So, really, if I can't face my own fears then who am I to talk?"
She said tried to find every excuse to get out of it. "But come morning I am going to be on that start line with my heart in my hands - or in my stomach."
The race is the only day of the year the harbour is closed to marine traffic, according to event director Scott Rice. The 10th anniversary has attracted a record number of entries.
"It's fantastic to see the way that not just Aucklanders but swimmers from all over New Zealand and further afield have embraced the opportunity," Mr Rice said.
The weather forecast is for near-perfect conditions, with fine weather and a five-knot breeze expected. High tide is at 7.54am.
• Swim starts from Bayswater at 7am.
• Swimmers divided into six waves, based on ability.
• Record field of 1,700 entries.
• Fastest time expected to be around 33 mins. The slowest time will be 1.50 mins, after which swimmers are removed from the water.