"I'm certainly looking forward to it," Mr Marshall said.
"I can remember my first [ocean] swim very well. It was the Auckland Harbour Crossing. I can only breathe from one side so I was scared the bad weather that was forecast might have come from that side - I would have been history. Fortunately, it was coming from the other side but I was pleased when it was over," he said, chuckling.
Mr Marshall will compete alongside 12-year-old Talitha McEwan this weekend in swimming the "I'm Going Long" leg of the race.
The athletic youngster is the youngest competitor doing the long swim and has waited a long time for her chance to take part, even if it will be "a walk in the park".
"I just really enjoy swimming and I just want to be a part of it and feel like I just have to push myself. I'm really happy when I finish," she said.
Talitha said she enjoyed doing long-distance swimming. "I'm just better at it."
Mum Veronika McEwan said Talitha started short-distance swimming when she was 7 years old.
"When she was 10 she wanted to step it up. That's when the rules changed and under-12s weren't allowed to do long swims."
Talitha busied herself with long-distance swims all over New Zealand instead, biding her time before finally being able to take part in Saturday's event. She also gets up at 5am every morning to get training in before school.
McEwan said her daughter filled her with inspiration, as did people like Mr Marshall.
"There are amazing people in their 80s and 70s are taking part and quite often they do better than 30 to 40-year-olds," she said.
Event director Scott Rice said the event was one of the most popular in the seven-swim season, attracting people from all walks of life.
"This year we have a huge range of swimmers, from youngsters who are setting their sights on their first big swim to those smashing a personal goal, to some more seasoned swimmers who do it every year and that's the great thing about this type of event, anyone who puts their mind to it can cross that finish line," said Mr Rice.
Generation Homes Sand to Surf:
When: Tomorrow, staggered starts from 10am
Start / Finish: Mount Main Beach
Distances: 200m OceanKids, 500m Give It A Go, 1km Step It Up, 2.6km I'm Going Long
Age range: 7 – 82 years old
Fastest 2017: 30:27minutes – I'm Going Long