On Monday, five years later, he is returning to finish the run.
The Wellington marketing professor will be joined on part of the run by three surgeons who saved his life, and will be accompanied in other sections by numerous people who helped in his rescue and recovery.
He was on the phone to his wife and two daughters on August 2, 2017 when he saw the dark-coloured ute approaching.
He watched as the ute crossed two lanes, before speeding up and veering onto the shoulder of the road to hit him.
"In that split second I knew I was going to be hit, so attempted to jump out of the way across the small metal railing, but I got slammed on the side," he said.
Ashill was flung into a ditch where he was not visible from the road. If it hadn't been for his family hearing what happened, he likely would have died.
"Sadly they heard the impact and my screams; that wasn't very nice," he said.
It took about 50 minutes for rescuers to find Ashill.
It has taken 16 surgeries and nearly five years to recover from the huge trauma his body suffered, which included open fractures, a shattered pelvis, and internal injuries from the bone fragments.
For several years he battled with "huge anger and resentment" as well as "massive grief" and guilt that his loved ones had to deal with such a traumatic event.
"Most of that is now gone," he said.
The driver who hit him was never caught and the case was declared closed about a year later. Ashill is confident the act was deliberate, and wondered why for a long time. He now feels indifferent towards the mystery driver.
His recovery, mental and physical, has been long and difficult.
"You take a couple of steps forward and 10 backwards, and you realise the limitations of a body that's essentially been rebuilt from the waist downwards, but the human body's an incredible thing. It adapts; weaknesses are compensated for."
He was told he could have the rod, screws and plates removed from his leg, but it would mean he could not run again.
"I'm not quite there yet and I don't know whether I will get there," he said.
He knew he always wanted to finish the run across America, which he had been doing in honour of his mother, who died of pulmonary fibrosis in 2015.
He began planning his return from his hospital bed, but it has taken until now to make it possible. He resumes the run tomorrow and expects to finish about June 18. He has about 1200km left to go, having already run almost 4000km before the ute hit him.
This time, he will have a GPS tracker on him so his support crew will know where he is and he will know where they are.
Before he was hit, Ashill said the first part of his run "became like a drug".
"I loved it, I met some amazing individuals along the way."
He also came across rattlesnakes, was stalked by two coyotes, and encountered a "very large bull" that blocked his way until a passing car helped move it off the road.
Ashill is looking forward to resuming the adventure at 5.30am (EST) tomorrow.
"I'm very, very fortunate to have another chance to go back and finish what I started."