Law (50) has been preparing for the 7in7 Challenge through the Queenstown Lakes district for two years and has now raised more than $123,833 towards his $150,000 goal for the Leukaemia and Blood Foundation.
Law said he had struggled in temperatures of about 32degC and got cramp.
"Hot! Tell me about it. It is unbelievable," he said.
The experienced long-distance athlete is pacing himself to conserve energy for the days ahead.
He admitted he felt tempted to speed up just so he could finish sooner, get out of the heat, and jump in the lake at Glendhu Bay.
"But I didn't want to push too hard and suffer later," he said.
After a swim, massage and food he was feeling much better and preparing to go to Makarora, where he starts the Young Wilkin Circuit this morning.
Law said the Wanaka charity auction on Saturday, with guest auctioneer and former All Black Marc Ellis, had been fantastic.
It attracted a large crowd and raised $10,365.
Ellis ran a half marathon yesterday but proved too sprightly for Law.
"I tried to keep up with him for the first kilometre but I had to let him go," Law said.
Today: Young Wilkin Circuit, Makarora, 55km.
Tomorrow: Motatapu Gold, Wanaka to Arrowtown, 49km.
