Most finishers complete the event in 11 days, with the fastest of the elite athletes finishing in under eight days.
Harper is no stranger to endurance cycling - the 41-year-old cycled the length of New Zealand, completing the 2170km from Cape Reinga to Bluff in four days, nine hours and 45 minutes and setting a new world record in doing so in 2017.
In November, he won the Maxi Enduro, which is part of the BDO Lake Taupō Cycle Challenge, with a time of 21 hours, 45 minutes and 23 seconds. The Maxi Enduro is a 640km ride - four times around Lake Taupō.
"I always feel like I need something to test yourself," Harper said.
But this coming challenge, which is one of longest running ultra-endurance events in the world, is one Harper - a former national rower - knows will really test his limits.
"This is going to be so massive, it's such a big commitment ... I'm looking forward to it."
The timing of the event means Harper will cross the US, including across the desert at the beginning, in peak summer - conditions he is not accustomed to.
"The race I'm doing, one of the biggest factors is the heat in the first two days," Harper said.
"Over half of the field don't finish."
Harper decided about a year ago he wanted to commit to the Race Across America and has made a lot of sacrifices to do so.
"I finished working at Christmas to just be able to apply myself to this properly ... there's a lot more to it than just riding a bike. It's more of a mental challenge, not just physical."
By stopping work he is able to train during the day, while also spend time with his wife and daughters, aged 9 and 11.
"If I was still trying to work I'd only be working 60 per cent, training would be suffered and family time would be suffered, everything will just be stressed. This way I can train in the middle of the day, in the mornings and night do stuff with them and help out."
Now, he has just over 120 days left to get used to it, before beginning his journey in California and cycling his way through Arizona, Utah, Colorado, Kansas, Missouri, Illinois, Indiana, Ohio, West Virginia, Pennsylvania and Maryland.
In order to prepare his body for expected hot conditions, he travelled to Tauranga this week for a 24 hour training session in the Adams Centre for High Performance's heat chamber as part of sponsorship through the organisation.
The session involved cycling for three hours at a time in 40 degree heat, with plenty of technical monitoring throughout. His breaks lasted between 20 minutes and about an hour.
Harper said the session was "different" to being out on the road - and not just because of the heat - with the biggest barrier being having to stare down at the handlebars and not being able to occupy the mind.
"All the sessions were tough to be honest."
"I struggled a little bit with the lack of visuals ... not having a road. It's quite different, visually not having that stimulation."
But he got through it, describing the experience as beneficial, providing "essential information" through the monitoring of oxygen levels, blood pressure, body temperature and more.
He says he got exposure to hot conditions so he didn't have to face it for the first time on race day, and more.
"Any exposure is going to help," he said.
"You get quite a bit of information.
"The guys here were here all night."
Harper is looking forward to June's big race and while he is obviously a person who constantly wants to push himself further with a new event, at the moment he's only focusing on this race.
"The first goal is to finish safely and within the timeframe.
"I'm a competitive guy, I'm going to be as fast as I can and try to be competitive at the front end of the field."
He'll be travelling with a support crew of nine people and it's an expensive exercise that will cost about $120,000, he says.
The crew will help with logistics, bike maintenance, nutrition, health and navigation.
"Logistically it's just massive, especially travelling to the other side of the world, just with vehicles and gear. It takes a lot of planning, I've got a good crew helping me to piece it all together."
