The Owaka-based relief teacher is combining work, helping out on the family farm and other commitments with an intensive training schedule as his first foray into the biggest event on the New Zealand Ironman circuit nears.
It is a long way from the kid who enjoyed playing hockey and running cross-country at school but Cullen has always enjoyed running and being in the outdoors, so it was obvious the world of multi-sport would appeal.
But it was not until he attended the University of Otago, when he bought a bike, that he began thinking about taking part in duathlons, triathlons and multi-sport events.
Like many who indulge in this tough but rewarding pursuit, Cullen said multisport was "highly addictive".
His most gruelling events so far have been the Coast to Coast, in which he has competed three times.
His team finished fifth in its category at this year's event.
The popular Goldrush event was also another event he has contested regularly.
But Taupo is by far the biggest challenge.
The March 7 event has come around very quickly and Cullen has upped his training schedule as he does everything he can to be well prepared for it.
Two daily sessions during the week, totalling up to three hours, are mandatory while he lifts his work rate at weekends, sometimes spending up to twice that time running, cycling, swimming, surfing, kayaking and doing any other physical activity he can think of.
Mixing up his training is the best way to avoid boredom and the threat of no motivation, he said.
"If I try different things in training, it makes a bit less boring and keeps me interested."
But the physical hurdles were not the only ones facing him and many others.
The sheer cost of buying equipment and entering races was also a factor.
The Ironman entry fee is $750 before travel and accommodation expenses, making it a very expensive exercise.
Cullen said his parents, Neil and Pam, were "incredibly supportive" and they often helped out with the costs involved in multisporting.
"It was only after I graduated and started earning some money that I could serious about it, to be honest."
The addictive nature of mullti-sport was enough to keep him more than interested in continuing, he said.
"You can do all that training and think you would be keen to stop it after a race but after about a week, you start to miss [the training] and you get out there again."
He recently completed a 150km bike ride and "mixed it up" by completing a 20km run, followed by a 50km bike leg.
