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Originally, it involved 13 people on a 50km or 80km run through the Naseby Forest in freezing August temperatures, but it has evolved to cater for the most dedicated of trail runners, incorporating 100km, 200km and 100mile (161km) races.
The latest event upped the stakes with the addition of a 200mile — or 322km — race.
But with the completion of the 2021 race at the weekend — five months late after it became a casualty of the August lockdown — the couple have completed their final lap.
The event will continue to be run by the Naseby Development Charitable Trust.
It was the right time to hand the race on to the Naseby community, Mrs Sinclair said.
"It feels good but sad.
"It’s time and we both realise that," she said.
Her husband agreed.
"[The race has] evolved over a series of years and was a strict learning curve, but it’s been a hell of a ride," he said.
"The Naseby Development Trust are going to take it over.
"We’ve always maintained that we hoped that something like that would happen. It’s what’s happened with the Kepler [Challenge] — they started with someone running it and then a committee took it over and it’s a huge money-spinner for the town. We see the same likelihood of that happening here for Naseby."
Trail running was a great equaliser and that was evident in the way runners supported one another during the race, Mr Sinclair said.
"Race doesn’t matter.
"The only race that matters is the running race and, you know, cultural beliefs or religion or whatever — they don’t come into it. It’s a melting pot, it really is, in lots of ways — socially, economically. There’s some people here on the bones of their arse and there’s some people here who are company directors," he said.
"Our philosophy on the race was for everybody to finish," she said.
"It doesn’t matter where you come, whether you’re first or last — sometimes the person who came last is more important than the person who came first because they’ve been out there doing it longer and they’ve probably got bigger gremlins to overcome.
"Everybody’s an equal, just doing different distances."
While the couple are passing the baton, they will continue to be involved behind the scenes, and Mr Sinclair intends to return to run the 100mile race in August.