Dr Mitchell (40), a lecturer at the University of Otago's tourism department, had his pelvis smashed into seven pieces and his urethra torn in half when a truck and semi-trailer ran over him.
Taken to hospital "fully conscious" and in excruciating pain, Dr Mitchell says he was lucky the truck missed his vital organs.
As it was, his injuries caused his lower body to swell up to the size of his hospital bed, before tubes were inserted to drain about 10 litres of fluid from inside him.
"I was told I wouldn't be able to walk again.
"Sports? Totally out of the question," he said.
Dr Mitchell, 30 years old at the time of the accident, spent 65 days "flat on my back" before he started rehabilitation to "learn how to walk all over again".
He underwent 12 operations during a 13-month period, as surgeons tried to repair the damage wrought by the 10-tonne truck.
He still has no feeling down 20% of his right side.
Two years after the accident, Dr Mitchellreturned to Dunedin and "did nothing except feel sorry for myself".
In 2008, depressed and weighing 135kg, his doctor advised him he needed to change his lifestyle.
He began running.
Five kilometres a day at first, and then further and further.
He completed his first half-marathon in Dunedin later that year and has run 15 more, including the full 42.2km distance at the Southland marathon last year, shedding 35kg as he worked to get himself into shape from training.
Not long after committing to his fitness regime, Dr Mitchell decided he would also start training for the Challenge Wanaka half-triathlon - a 1.9km swim, 90km cycle, and 21km run.
Dr Mitchell heard about Blenheim girl Jacqueline Wyatt (12), who had been run over by a truck in November last year suffering similar injuries as he had, and resolved to do something for her.
He upped his training to go for the full Challenge Wanaka and also set up an online pledge page to raise funds for the Wyatt family.
He completed the Challenge Wanaka on Saturday in a time of 15hr 07min 13sec and has raised more than $1500 for Jacqueline, whom he will meet for the first time at Christchurch Hospital, later this month.
"I've been in touch and it will be really good to meet her.
"I'm wary of over-promising because everyone recovers differently, but it is all about giving her hope," he said.
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