My favorite place to get in a good long run on Oahu is definitely the Ala Moana Beach Park.
You run up and down Ala Moana, around Magic Island, and then back the same way.
Altogether you can get in a good 8-10 miles. It works well going in either direction. It features great views of the Honolulu skyline as well as of Diamond Head. The route is all on wide sidewalks with absolutely no vehicular traffic to contend with.
Happy running!