And his next one - 24 hours spent running, walking and ''crawling'' up and down Mt Iron on the outskirts of his home town - fits that bill perfectly.
His ''24 Hours on Iron'' challenge will take place on the 240m-high Wanaka landmark from midday this Saturday.
It is a forerunner to Law's most ambitious goal yet, his ''High Five-O Challenge'', in which he will run 50 off-road marathons on New Zealand peaks in 50 days, starting on February 7 next year.
Both challenges will raise money for the Mental Health Foundation of New Zealand.
Law has previously raised almost $300,000 for various causes, having twice completed New Zealand's seven mainland Great Walks in seven days, run the equivalent altitude gain and loss of Mt Everest in one day and run the 1014km length of the South West Coast Path in the United Kingdom in a record time of 17 days.
''I do kind of like these silly, mad challenges ... they're good for giving me a bit of motivation and ... they make a good story, so it does help with getting the word out.''
Law does not expect to run for the entire 24 hours on Mt Iron and is not even ''100%'' sure he can last the distance.
''But I guess that's what makes it a challenge. If I knew I could do it there would be no challenge to it.''
Running for up to 25 hours a week should stand him in pretty good stead, though.
''Since 2009 in fact, when I did my very first multi-day fundraising run, I've put a lot of miles on my legs and I've built up a lot of experience, so running long distances is kind of second nature to me now.''
Moreover, Law's mental stamina was his greatest strength.
''Probably more than anything what gets me believing I can do it is knowing I'm a real pig-headed person and I don't give in easily.
''The fact of the matter is it's very much a mental game. You can only prepare physically so much and the rest of it is just about being tough up top here, so hopefully I've got that going for me.''
Law hopes people will join him for some of this weekend's Mt Iron mission, to help keep him ''mentally fresh''.
People can also get involved by guessing his total vertical gain over the 24 hours. Each $10 donation to his fundraising page, www.high50.org.nz, earns one guess.
Law's previous challenge - climbing the height of Everest in a day - took him 24 hours and about 45 minutes. He hopes to complete a similar vertical gain this weekend within the 24 hour time frame.
During next year's High Five-O, the shortest distance he will cover in one day will be 25km - albeit up Mt Taranaki, while the longest day will be about 61km running the Kepler Track and Mt Luxmore.
