Walking 5 miles every day on a treadmill, in addition to daily walking? Wow! Try it and you will see that it has an effect, fish oil or no fish oil. I don't have a treadmill in my home, so I'd have to go to the gym every day. If you really do some activity like that EVERY day, you will see very good results, without changing other things. I know someone who lost 100 pounds walking 10 miles every day. She became VERY active. The weight dropped off. She ate carbs, carbs and more carbs, too!
It takes me an hour to walk 2.5 miles, really a 'stroll,' not a power walk by any means. Think how long it would take me every day to rack up 5 miles every day, and 10 miles every day. But what results!