They started the hard alpine track from the Blue Pools at 6am and finished together at the confluence of the Makarora and Wilkin Rivers about 11 hours and 45 minutes later.
Law was receiving medical attention and had a drip in his arm last night as he spoke to the Otago Daily Times by phone.
"I think I got pretty dehydrated [on Sunday, during the Twin Lakes marathon] and felt pretty awful for the first six hours today ...
"It was a pretty dark morning.
"I wasn't having a lot of good thoughts about the whole thing," he said.
After reaching the exposed Gillespie Pass, the highest part of the route at 1629m, the effort became easier and he recovered during the afternoon.
Law attributed a caffeine sports supplement and his supporters help for keeping him going.
Richard Ussher, of Nelson, carried his pack for several hours when things got really grim.
Law admitted he had worried the record high temperature of 32degC on Sunday had taken more out of him than he wanted, but he thought he had rehydrated and rested well before yesterday's mission.
Conditions were fine and a slightly cooler 26deg yesterday, and the weather pattern is forecast to continue today.
Today's run over the 49km Motatapu track is another testing alpine route, with the highest point at 3400m.
This is the equivalent of climbing Aoraki-Mt Cook from sea level or Auckland's Sky Tower 18 times.
Law has estimated he and supporters could take at least 12 hours to complete the route.
Tomorrow, he tackles the 38km Wakatipu Wonderland along the shores of Lake Wakatipu and over the Ben Lomond Saddle at 1316m.
Law has raised at least $123,000 for the Leukaemia and Blood Foundation during his 7 in 7 Challenge.
He hopes to raise $150,000 by the time he has finishes his week of pain in Te Anau on Saturday.
