Seltjarnarnes, to the west of Reykjavik has some really nice walking and running paths. Some people claim it is sometimes windy there but not us !
The running club in Seltjarnarnes hosts a 7.5 and 15km race in may every year. The routes here are used in that race.
They are the same except the longer one also goes around the local golf course.
[googlemaps https://www.google.com/maps/d/embed?mid=z0S-oaoW3W4s.kFPPS_dvjJ5M&w=640&h=480]