I Run 30 To 40 Minutes On My Treadmill Everyday With The Speed Of 6.5 Miles Per Hour - Is That Good Cardio?

I mean - does that contribute well to my weight loss? I have a small belly but still and would like to loose it so I can work on getting a sixpack. . . Also, is that good stamina training?
Running outside is something I won’t do, as there is nowhere to run to and I don’t really want to. . . I’ll add that I’m 15 yrs old if that helps.