A field of 161 runners took part in the event yesterday, which also included 10km and 5km races.
Armstrong opened up a sizeable lead early on in the half-marathon race along the Hawea River track to run away from the chasers and win in a time of 1hr 16min 30sec.
Armstrong finished more than six minutes ahead of his nearest competitor, Geoss Williamson, of Dunedin, who recorded 1hr 22min 38sec.
Helles won in a time of 1hr 37min 28sec along the 21km off-road course and takes a two and a-half minute lead into the three-race series for the Pawson Trophy.
Whitney Dagg, of Dunedin, was second woman home in 1hr 40min.
The John Pawson Trophy will be awarded to the fastest male and female runners, with the best cumulative times across the three-race series.
The next race in the Riverrun series is the 14.1km Clutha River trail from Albert Town to Luggate on February 19, followed by the 14.3km final race from Glendhu Bay to Wanaka on May 21.
