The 21-year-old Redcliffs resident ran the 322km in 57hrs 54min, completing 32 laps of the Naseby forest in Central Otago.
Watts was the second woman home and sixth overall in the race.
She is the only woman under 30 to ever complete the course.
But age is no barrier for Watts.
“Just because you’re young doesn’t mean you can’t do these big races.
“There’s just a blank spot for that under 30 women’s and I really wanted that to be filled,” she said.
The oldest person to complete this year’s course was 63-year-old Annie Hunt from Twizel.
In addition to trail running, Watts is completing a Bachelor of Commerce at Canterbury University and works two jobs - at a plant nursery and marketing for a painting contractor.
Watts got into trail running at 18 after a friend roped her into taking part in an adventure race.
She discovered she preferred the trail running aspect because of its longer distances and continued from there.
“It’s a form of meditation in a way,” she said.
Since then, she has completed the Coast to Coast three times and run the Northburn 100 Miler, near Cromwell, in March, which gave her the confidence to have a crack at the Naseby event.
“I was running with two friends and they said if you can do Northburn, you can do Naseby.
“I hadn’t really thought about it until then. From there, I just went all in,” she said.
Watts also took up the challenge to fundraise for the Canterbury Westpac and West Coast ROA Mining rescue helicopters as part of the ‘Back to Base’ challenge.
"They’re a huge help to the whole running community. When you do get into trouble, knowing that they’re there for you, it’s just a really great safety net.
"I just wanted to give back a bit to them for the amazing work they do,” she said.
Watts ran 503.71km and raised $505 over the month of August.
She averaged 70-80km a week, with some training blocks up to 130km.
The key to her preparation was having a flexible schedule and a balance between big training blocks and rest.
“It’s not always running, sometimes it’s just tramping in the hills or hiking ... enjoying being outside and enjoying what you’re doing. I think that’s super key,” she said.
The magnitude of the task ahead started to sink in during the lead-up.
“It’s further than I’ve ever gone, so there’s a lot of not being sure what to expect.
“But I went into it knowing, there’s a job to do and we’ll get it done,” she said.
Watts credited her support team - parents Jo and Warwick and friend Lottie Moffat - who attended to her every need.
“They were medics, they were nutritionists, they were photographers, they were everything,” she said.
When the race started she grappled with the idea of how long she had to go, but made a goal to try and smash out the first 100km, which she did in just over 24 hours.
“That’s the business end done, and then you’re like, right, let’s just look after yourself and get the next hundred done,” Watts said.
Fatigue set in sporadically throughout the course, but mental fortitude enabled Watts to push through the pain.
“You’ve just got to ride out the lows and you will get the highs,” she said.
Her support crew was stationed at a particular point on the course, but Watts avoided staying at base for too long.
She picked up nutrition bars when she needed a boost and only stopped for two 20-minute naps during the almost 58-hour run.
By the 310km mark, Watts was blistered and exhausted, with the distance she had run starting to catch up with her.
But once she reached the final 10km, her achievement began to sink in.
“You feel like it’s never going to end and then all of a sudden you’re there, it was pretty surreal,” she said.
Watts is eyeing up another miler early next year and is tempted by the challenge of a backyard ultra.
