And after the ceremony, they - and many of their wedding party - will hit the road again for another three hours' running to the finish line.
The couple are Australians Mark Smoothy (52) and Alyssa Coe (41), who arrived in Wanaka on Wednesday.
Eighty-five guests have accompanied the couple from Australia, 11 of whom are competing in the 226km Challenge event.
Mr Smoothy told the Otago Daily Times the couple were trying to restrict the ceremony to about 20 minutes ''because if we stop too long, we might not want to get going again''.
Mr Smoothy will wear a specially designed triathlon ''tuxedo'', Ms Coe a triathlon ''wedding dress'' with the wedding party in similar styles.
On a more traditional note, the wedding march will play as Ms Coe is escorted to the altar by her father, John. Her mother, Susan, will act as matron of honour and bridesmaid.
Wanaka celebrant Bev Clarke will officiate.
Mr Smoothy said he and Ms Coe met through an internet dating site and have been together for two years.
''How many girls do you know in the world that would get married in a bloody long-course triathlon?'' Mr Smoothy asked.
''She's pretty unique, mate.''
Ms Coe, originally from California, said it had been quite hectic planning a wedding in Wanaka while training in Brisbane for the race - her sixth long-course triathlon.
Mr Smoothy proposed during a Melbourne Ironman event last year and he believed their wedding would be the first between competitors taking part in a triathlon.
They hoped all the wedding party would make it to the ceremony by 6pm, although members of the bridal party noted Mr Smoothy was often the one who was late to events.
Mr Smoothy, who has competed in all previous Wanaka Challenges, said he had been warned by his bride about the run home after the ceremony.
''She said to me: `Don't get any bright ideas about wanting to consummate the marriage on that last 20-odd ks'.''
