He said it was a massive feat of endurance that had never been previously attempted.
True to his strong belief that "ordinary people are capable of extraordinary deeds", Mr Law has already exceeded his initial goal of raising $250,000 for the Mental Health Foundation and has now upped the stakes to $400,000 by the time the challenge ends.
Running as a supporter of the cause today is the ultramarathon founder, Rotorua's Paul Charteris.
Mr Charteris said he'd battled depression in the past. In a blog as part of the High Five-0 fundraising page, Mr Charteris spoke of contemplating suicide during his worst time, being put on academic probation, being diagnosed with depression and what helped him out of it.
Mr Charteris said speaking out at a forum about some of the most private parts of his life including not really knowing his father was a turning point.
"[The] greatest lesson from those three days is all about the importance of sharing your life with others."
Falling into trail running, Mr Charteris said he found an activity and a group of people he could pour his energies into.
Getting out of an environment that was toxic to him - the university environment - was a big part, as was sharing his life with others, getting fit and changing his diet, he said.
Mental Health Foundation chief executive Judi Clements said Mr Law was a great ambassador for mental health who gave generously of his own time, and was also hugely supportive of others.
"While not everyone wants or is able to test their limits in the way Mal does, he's inspiring people throughout New Zealand to be more conscious of their mental health as well as their physical wellbeing and helping to reduce some of the stigma and discrimination that can surround mental illness.
"The Mental Health Foundation is enormously appreciative of this sensational fundraising effort. It will give a huge boost to the work we do to create a society where everyone can experience positive mental health and wellbeing. And we'll look forward to celebrating with Mal on Day 50."
Mr Law said he was ready to throw everything he's got at the Partners Life High Five-0 Challenge.
"Regardless of the weather or how tired and broken I'm feeling or of the inevitable pain and discomfort, I will not quit unless I really have no other option," he said. "And even then I won't guarantee it. Having a cause I'm passionate about provides me with massive amounts of motivation to keep going when things get tough."
To make an online donation visit the website, www.high50.org.nz or text "GOMAL" to 2446 to donate $3. To read more of Mr Charteris' journey go to: www.high50.org.nz/blog/mental-health/finding-me---paul-charteris/