A sore quad in the past two months makes her a tad anxious about pulling up today but she likes to think she's in a similar mould to last year.
"I normally win," Muir says matter-of-factly.
In the past 12 months she has claimed the bragging rights to the Kepler Challenge, near Te Anau in Fiordland, the Wellington Marathon and the Hounslow Classic in the Blue Mountains of New South Wales in Australia.
Muir, from the Coromandel, settled in Napier five years ago after meeting Kristin Day, a primary teacher, who was then studying at the Eastern Institute of Technology.
The married couple are Napier Harrier members but Muir hopes to do more runs along the Ahuriri waterfront if everything follows the script with the demands of work and racing.
She isn't too sure where the seduction with running, let alone 102.7km of trails and forestry roads, comes from as she tries to conquer today's race, which makes its way from Rotorua to Kawerau in the Bay of Plenty.
"I was always sporty but running wasn't a sport in the schools I went to, where it was compulsory once a year on athletics day."
However, she ran whatever was in front of her and always did well without training.
"As a kid I played soccer and surfed. We lived in the bush so I spent a lot of time outdoors being active."
In her late teens she "started running for myself before and after school". The then 17-year-old didn't have any competitive ambition and did it purely for enjoyment.
"I was having a lot of stress in my life at the time so it was a good mechanism."
It was about that time she visited Mt Ruapehu to watch an annual event, The Goat, and her competitive juices began to flow.
"That was, sort of, my first real race and I won it. I was a junior but I won it overall so people told me I should take it a bit more seriously and do some training.
"I didn't have any concept of training because I used to just run up and down our driveway, which was a long gravel road."
Her parents, Stephanie Muir, a nurse in the Coromandel, and her father, the late David Elliot, who was a wedding photographer, were devoid of sporting genes but, nonetheless, happy for her.
They were accustomed to her steely resolve but amused she found traction with running.
"My mum's not too impressed with the 100ks. The first year I did it she didn't approve and felt it was bad for me," she says, mindful it doesn't help that in ultramarathon there's a culture of an exaggerated sense of power.
"They like to show off about how hard-core they are and tell lots of gory stories.
"In my experience, if you train right and do it smart it's a race like any other."
A grinning Muir reveals she doesn't have any harrowing accounts in her resume but she has had a couple of bad races.
For instance, in 2014 she was competing in the 80km event of the Ultra Du Mont Blanc in France where altitude sickness got the better of her. "It wasn't that high but high enough because I started to feel dizzy and queasy and couldn't think very well so I fell a couple of times and everything just fell apart."
She attempted to pull out but friends at a drink station stuck a sickly sweet fizzy drink under her nose and coaxed her to carry on.
"I did another leg of the race and ended up walking it out."
Her times are relative to myriad terrain. For argument's sake Tarawera is up and down but the hills are rolling whereas a more mountainous track will put extra demand on her reserves.
"I guess I'm averaging 11km an hour which isn't that fast, you know. If I go for training it's about 15km an hour."
The other variables, such as stopping at three to four of the eight stations at Tarawera to refuel, tends to eat up about half an hour so it's imperative to carry out transitionary phases smoothly.
She loves to run to music amid scorn from some quarters that she needs to soak up the stunning Blue Lake, falls, dotted rocks and rivers meandering through native bush.
"I enjoy both but I like the music during the harder parts to motivate me and towards the end when I like to listen to some noisy, angry music," Muir says, opting for slower numbers at the start line to soothe the jangled nerves as she will confront nearly 3000 vertical metres of climbing and even more descending.
The heavy metal music at the end is an ideal distraction from the spent muscles and the near-empty motivation tank.
"Feeling angry is like the last thing you've got left," she says with a laugh, realising it's an emotion she can draw on in the blink of an eye "to feel like I'm badass".
That is not to say Muir doesn't let her mind wander while taming the terrain.
It can vary from what she needs to add to the magnetic shopping list on the refrigerator door to impending social engagements but work isn't up there.
"Sometimes I think about nothing and that can be great as well. At times you realise you've done almost 20ks so it becomes like meditation."
When the fingers of fatigue wrap around her brain she finds comfort in her wristwatch for mathematical stimulation on estimated time of arrival to the next food and beverages station.
"A kilometre and I have to do it all over again because I have to keep my brain busy somehow.
"The worst is when I start counting my steps because I then start thinking of something else," she says with a laugh.
It's godsend that in the last 40km runners can have pacesetters to keep them company although companions aren't allowed to carry any load for the runners.
"Kristin will do that with me so I'll have something to talk about," Muir says but mindful any old banter won't do.
"Sometimes if I initiate it it's okay but usually I'm tired and cranky so if he tries to bring things up I tell him to shut up."
Sister Rose, 27, is a casual runner although she suspects their mother ran, but not competitively, in her 20s.
"She jogs a little. She's not a runner. Sorry Rose," Muir says apologetically of her sister, who lives in the Coromandel.
Her love of technical and off-road running doesn't encroach the glitz and glamour of the Olympics at present. "My ability to navigate the terrain gives me an advantage over people who might be fitter than me on a flat course," says Muir, who only ran her first marathon (2h 47m) last year, in Wellington, after attempting another several years ago but failing to finish it.
"New Zealand qualifying time for [the Olympics] is under 2:30 so I have a lot of work to do," she says. She doesn't feel it's unattainable but she'll have to find a coach "if I want it bad enough".
"It's hard to be good at both. You can be quite good at both but not excel in either," she says, enjoying doing both and, more importantly, having fun.
Ironically it was her husband who ventured into the 100km race first and she manned his stations before the dramatic role reversal.
"He's finished with that. He thinks it's too long so he's trying to get faster for marathons, half-marathons and things like that."
Her mother and Day will be part of her support crew today.
While her training is regimented she tries to fit it in where she can rather than attempting to emulate the feat of elite runners.
Injuries aside, she's clocking up about 150km a week in training cumulatively over 10 hours, with weekends allowing for longer distances.
Come the race today, though, her sensible diet will go out the window so she can find a modicum of harmony with her biological constitution.
"I really eat bad food when I'm racing. The worst thing for me about these races is to get the food in because it's really hard to eat when you're running.
"If I try too hard I can start feeling sick or get a sore stomach so the normal thing people eat are sugars because you can digest them easily."
Nevertheless, she even ingests sports gels quite sparingly, diluting the syrupy paste with water to beat the sickly sweetness.
Nut bars tend to help settle her digestive system by providing a base although not necessarily the best source of energy.
"What else do I like to drink? Ah Coke, which I don't drink normally but at the end of a long race it makes a big difference," says Muir, drawing sustenance from the overload of sugar and caffeine.
Incidentally, just the very thought of people perched on couches soaking up fizzy concoctions without much physical activity makes her cringe.
