The previous day, the 18-year-old finished the 200km ultra adventure race The Revenant, running for 59 hours across high country farm Welcome Rock, near Garston.
The fifth person to finish the annual event since its launch in 2018, he acknowledged it was "pretty special" to complete it.
The race requires runners to navigate four 50km laps of the farm and climb 16,000m.
Competitors were prohibited from using any electronic devices and had 60 hours to finish.
"You’re allowed a compass but no watch and no alarm clock, so you’ve got to be creative if you want to wake yourself up from a sleep," he said.
At one stage, he had 20 minutes’ kip one night and relied on the cold weather to wake him up.
He got another 10 minutes of sleep in a woolshed at a race checkpoint.
If any of the 39 competitors failed to finish a lap in 15 hours, they had to "tap out" of the race by touching a bottle of Revenant whisky.
Tradition states those who finish the race down a shot from the bottle.
Fynn expected the sense of relief of finishing the race to be greater.
"It is not as big as you think because you are so fried and you’ve been moving for so long that you’ve gone into a rhythm and it feels weird to not be moving."
Remaining still after crossing the finish line was a strange sensation, he said.
"You haven’t slept and you’re not quite right in the head and you feel like you should be getting up and going again."
The sensation took about half a day to go away, he said.
Fynn was raised on a sheep and beef farm in Lumsden, and had recently finished studying at Southland Boys’ High School.
He was set to start studying for a bachelor of commerce degree, majoring in supply chain management, at Lincoln University.
During his summer break, he had been working in a shearing gang, but missed work the day after the event.
‘I was meant to be crutching on Monday because I didn’t think I had a realistic chance of finishing the race."
After finishing the event, he called his boss seeking to spend Monday resting instead of crutching sheep.
He returned to work on Tuesday last week, but did not set any personal bests in the shearing shed.
On his breaks on his first day back, he talked to several media outlets, including a live cross to Breakfast from inside a car on a farm near Otautau.
At The Revenant, he crossed the finish line with Darron Jones, 44, of Nelson, who is the sixth person to complete the race.
"I knew him to say gidday and we were the same speed and worked well together. It was bloody lucky I got to do it with him because you would get sick of yourself real quick if you were out there by yourself for 60 hours."
The next challenge in his sights was Expedition Ozark, a five-day adventure race in Arkansas in the United States from April 7.
"I’m keen to go. It is just whether it fits in with uni — but it should fall in the mid semester break."
