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The Three Peaks Mountain Race will be run for the 36th time over one of the city's most challenging courses. A 26km trail begins at Ross Creek and finishes at Chingford Park, returning to its original course after the completion of work at the reservoir.
A total ascent of 1370m takes runners up Flagstaff, Swampy Summit and Mt Cargill. They will race along muddy tracks and through regenerating forest and tussock-topped peaks.
There have been 152 entrants confirmed, although runners can register up until the race starts.
Favourites among local competitors are Tim Bolter, Andrew Lonie and Sharon Lequeux.
The race starts at 9.30am and usually takes about two hours.
That mark has been broken several times and in 2010 Dougal Thorburn ran 1hr 55min 44sec to set the record. Sarah Chisnall holds the women's record of 2hr 19min 56sec.