At the time I consider myself to be at by best cardiovascular health (at age 17), I could run a mile in about fifteen minutes. (Or, that's about 6.5 km per hour). That was when I was on a good diet, was exercising for about 5-6 hours a day, and was extremely defined. (Was about 210 pounds back then, six-pack and everything).
These days, even though my physical strength is a lot higher than it was 11 years ago, my cardio health has dropped, I can run a mile in about 20 minutes now (or roughly 4.8 km per hour). Then again, I'm carrying about 50 pounds more on my frame. Not all of it's fat, but even the self-supporting stuff burns energy real fast, when I'm working out I literally can't eat enough to not feel hungry without rupturing myself.