"To see if I can. I haven't found what my limit is. Running 100 miles [160km] was not even close," she said.
She's also attracted by the fact she could be the first woman to finish.
Arnst is 35 and her job is managing the office of BPL Group for four days a week.
She has two children, aged 9 and 13, but has managed to spend 11 days in the Ruahine Range over the last three weeks.
She and Katie Wright were the first people to complete the 92km north to south Ruahine Traverse in one go.
They did it in 39 hours in December last year.
Previous attempts had all been thwarted by injury or weather.
Arnst and Wright have also traversed the full length of the Tararua Range twice, and were the fastest for one of the trips.
Wright, a GP, now works in Wānaka. She's attempted the Revenant twice and failed. Some people only try the first few rounds to start with, and competitors can be eliminated at checkpoints if they are slow.
Wright and Arnst will be among the 45 competitors next year. They may do parts of the race together, but will not be competing as a team.
"We wanted to not tie ourselves into that, because one of us might make it and the other might not," Arnst said.
Tramping the Ruahine Range is perfect training for the Revenant. The Ruahines are steep and the Revenant will have 16,000 "vertical" metres, over its more than 190km.
Map and compass will be the only navigation aids - no GPS or cellphones are allowed. It's in four timed circuits, with checkpoints that have to be reached in the right order. At each there is a box with a clue about where to go next.
It has to be completed in 60 hours, with sleeping en route. Arnst is envisaging "power naps".
She usually likes eight or nine hours of sleep a night. She'll start well-rested, and has been limiting her caffeine consumption so any caffeine she takes during the race will have maximum effect.
The route will take in tussock tops in Central Otago and Southland beech forest. It could be 30C, or it could be snowing. Arnst is used to being out "in some pretty gnarly conditions".
The start time hasn't been decided, and she has a suspicion it could be night.
"I suspect we will be starting in the dark, which means it will be slower."
She's been doing cross-country navigation sports like orienteering and rogaining (similar to orienteering) to prepare herself and she runs the Atene Skyline Track once a week.
She's a member of the Whanganui Land Search and Rescue team. She also leads trips for the Palmerston North Tramping and Mountaineering Club, but they tend to be too easy.
"Generally, I like to do more extreme things," she said.
Her progress can be followed on Instagram - https://www.instagram.com/ellys_explorations/
