But that once seemingly impossible dream will come true today for Mr Christensen (49), when he graduates from the polytechnic with a bachelor of applied science degree in physical activity, health and wellness.
"I firmly believe that it’s never too late — if it’s something that you really want to do, whether it’s physical or mental.
"I haven’t thought about the degree. It’s just starting to sink in," he said.
Mr Christensen will graduate in the second of two graduation ceremonies at the Dunedin Town Hall, starting at 3.30pm today — the first time the polytechnic has staged two Dunedin ceremonies in December.
A record 680 people will graduate in person, up from 550.
He had had his battles to complete his degree, mainly in trying to convince himself he could do it, rather than with the academic work itself.
And at times he had felt "very, very uncomfortable" during his studies, because he had stepped outside his comfort zone in terms of his previous life experience.
He had experienced a "massive transformation" in his life, and doors had opened up for him as a personal trainer.
After 18 years at Hillside, and a further year at Bradken as melt-shop team leader, he had decided to try something else.
In 2014, he had initially enrolled at the polytechnic for a two-year diploma in applied sport and exercise leadership, specialising in physical training and exercise prescription.
He was somewhat surprised later that year, when polytechnic staff approached him, pointed out that he was performing well and asked him if he would consider undertaking degree studies.
He was the first member of his immediate family to undertake tertiary study and although initially it had been unfamiliar, he had taken the academic work in his stride, Mr Christensen said.
He paid tribute to his wife, Karen, and polytechnic staff for their unwavering support throughout his studies, saying he could not have made it without them.