Robert Duffy, 57, will be among about 2000 of the world's best amateur and professional triathletes at the starting line of the 39th World Ironman Championship at Hawaii on October 8.

He is one of about 10 Australians to receive an invitation from the Ironman Legacy program to take part in the race that is regarded by triathletes as their "stiffest test".

Invitation recipients, numbering 125 across the world, must have competed in at least 12 ironman events but not in Hawaii.

Mr Duffy, a principal at DuffyElliott in Dubbo's Brisbane Street, chalked up his 13th ironman event at Port Macquarie at the start of May.

His first step on the road to the 2016 world championship was taken 17 years ago.

"In a common enough way I was 40 years old, unfit, unhealthy and stressed by work," he said when approached by the Daily Liberal.

"My wife and I took part in a motivational weekend seminar, out of which I committed to a healthier lifestyle.

"Initially, I was just wanting to be able to run around the block."

Within a few years he had progressed from short runs, half-marathons and marathons to triathlons, with help from family and friends.

"I was cross-training with a bit of swimming and my uncle Peter Duffy taught me how to relax in the water so that I could progress from 50 metres to one kilometre," Mr Duffy said.

"Then a friend of mine Michael Martin, who was a great cyclist, suggested if I was running and swimming I should buy a bike."

A heap of half-ironman races, each comprising a two-kilometre swim, 90-kilometre ride and 21-kilometre run, led him to his first ironman event in 2004.

"It took me over 13 and a half hours," Mr Duffy said.

"Now I can race around one and a half hours to two hours faster."

Mr Duffy's best result in an ironman event in Australia has been 11th in a field of 100 in his age group.

Competitors in the world championship on October 8 will have a maximum of 17 hours to cover more than 226 kilometres.

"Whilst the distances are the same at each ironman event, the Hawaii ironman is universally regarded as the stiffest test because of the conditions," Mr Duffy said.

"The swim course is in the open ocean where you are subject to ocean swell and currents.

"Both the bike and run courses are through the lava fields where the radiant temperatures are extreme and the bike course is also open to the trade winds which blow at 45mph."

The winners of age groups in more than 25 global ironman events along with a 100-member contingent of the world's best male and female professional triathletes are gearing up for the iconic race at Kona on the Big Island of Hawaii.

"Scott Stevenson of Dubbo is one of those incredible age group athletes who has qualified outright after a nine-hour-and-45-minute race at Ironman Australia," Mr Duffy said.

Mr Duffy will be decades younger than the oldest competitors.

"Usually there are people in their late 70s or even 80 who go around," he said.

"You meet incredible and inspirational people in this sport."

The solicitor will begin his 20-week physical and mental preparation for the big race after recovering from Ironman Australia.

"I will make sure that I am as ready as I can be," he said.

"I have been dreaming of and training towards getting my chance to test myself in Hawaii for over a decade.

"I am equal parts excited beyond words and apprehensive about the challenge.

"I receive incredible support from my family and my training partners in the local triathlon and running clubs.

"All of my focus will be on this one event. Once finished I will allow myself to think of what might be next."