A "nervous" Richie McCaw joined 100 multisport athletes on the first leg of a challenging endurance race in Wanaka this morning.

The first mountain biking leg from Minaret Station was one of seven stages in the Red Bull Defiance race, which had a combined distance of 142km and a total elevation of 5238 metres.

McCaw, who recently retired as All Black captain, was competing in this morning's 43km mountain bike leg and in a 25km run tomorrow.

Ahead of the event, the 35-year-old admitted that he was feeling "a little bit nervous."

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"I am just having a wee taste of the race, so I can't really claim too much today."

The rugby player said he might compete in the whole race next year.

"I was just watching the boys on TV doing the pre-season and I can't say I'm missing it too much.

"But I've got to do something to keep fit, so I'm excited. It is a good challenge."

Red Bull multisport athlete and race designer Braden Currie said the race would provide challenging conditions for competitors.

Currie was not competing this year, choosing instead to focus on the Rio Olympics in August.

"I think this is actually the first time for me watching a race, so it is quite difficult."

But he would "absolutely" be competing in the race next year.