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.
runninginiceland.com 