15 minute mile is my "walking while listening to fast music" fast pace. It is definitely a brisk pace but this is not even close to jogging. And 3.5 miles is my normal distance.

And even then, the point is more that if you are moving at 4 mph in your car, the car is doing almost no work. 4mph for a semi in shape human is perfectly reasonable to walk, so if that is all your car is providing you, it isn't worth it.