"I'm a firm believer in if you fail to prepare you prepare to fail," Harkness said.
"I definitely mentally prepared for it and I was determined to go to Tuesday if I had to. I had it in my head that it was going to be long."
It got easier as people dropped off and by the early hours of Sunday morning it was down two - Harkness and Heather Leeves.
Leeves checked out at about 9.30am when reaching for her jacket - one of only two to accidentally take their hand away.
"I think I was just really upset for Heather," Harkness said.
Eva Harkness won Ucol Whanganui's hand on car competition after a marathon 38-hour stay. Photo / Bevan Conley
"Both of them were just determined and fun and good to be around. You go through the same thing together."
Contestants were only allowed a 15-minute break every four hours.
"You could literally got to the toilet, sort out what you're going to eat and maybe sit down for a bit and then you're back. It was always quite rushed, it never really felt like a break."
Harkness said it was her mindset which got her through.
Mikaya Teki and Heather Leeves were two of 17 starters. Photo/ Bevan Conley
"I've thought about this for a long time and I never really doubted that I could do it and I think that's kind of key.
"Even the strongest give up and I remember thinking that quite a bit, even though I really wanted the other people to succeed as well obviously there can only be one winner."
Surprisingly the second night of no sleep was easy.
"Maybe your body gets used to the rhythm, and people dropping out, you get closer and closer," she said.
"So, it just got easier and easier really which is kind of backwards to how I thought it would be."
If anything, she said the hardest part emotionally was seeing her family's support.
"It's kind of the most family I'd seen in one place for a while. They were all doing things for me and I felt really appreciative and I felt hot and cold and tired and dizzy and I just really wanted to go home with them."
Runner-up Leeves said it was an awesome challenge.
"It's mentally hard to push through. When you are in it your kind of fighting your own need to do certain things."
Ucol Whanganui campus manager Bronwyn Paul said the competition was part of the institution's 10-year celebration, with Wanganui Motors donating the car.
"I'm just really proud of Eva and Heather. It's not easy."
As for Harkness, she had other plans before getting behind the wheel of her new car.
"I think I'm going to have spa and a mulled wine and a sleep."
