It turns out he was right.
The Hill City-University runner has just returned from Cyprus.
There he attended the Athletics New Zealand heat training camp for this year's IAAF world championships in Doha.
That comes after going under the qualifying time for the championships in his first marathon.
He ran 2min 15min 36sec on the Gold Coast to edge under the qualification mark by 24sec.
It was a great performance from Shields, who turned 31 this week.
The time does not guarantee him a spot for Doha though, as three other New Zealanders have also met the standard.
He will find out on September 6.
He said the camp in Cyprus had been good and he adapted to the heat well by the second week.
Now back home in Christchurch, where he is working as a physio after returning from Australia, he is happy with how the transition to the longer distance had gone.
"I've always seen myself as becoming a marathon runner, even since I was young," he said.
"My genetics work in favour of that. I've always had quite good natural endurance and natural running economy.
"I always planned on getting to marathons eventually, but through injury and having eight years of undergraduate study, it took me till I was 30 to be in a space where I could do one well.
"So it happened a bit later than I thought it would, but obviously it went well so that was good."
He plans to keep running on the track over summer.
That was beneficial for marathon running, particularly when it came to running economy and mechanics.
Despite his move to Christchurch he would continue wearing Otago and Hill City colours.
He attended King's High School, before spending three years at Purdue University in Indiana. Since his return to New Zealand, he has had success over 10,000m, winning national titles in 2013 and 2019.
He did not do so much club running nowadays, so did not feel the need to have to make the switch.
"I have a lot of pride in running for Otago and didn't really want to change to Canterbury.
"I'll probably stay registered with Otago while I'm a competitive runner."
He will keep training for Doha and if he makes that team, that will be his immediate focus.
From there, he may turn an eye to the Tokyo Olympics.
However, he said it was far more difficult to qualify for that - needing to go beneath 2min 11min 30sec to get there.
