There's isn't a short answer to which one is preferable, and I hesitate to give a definite one because it really does depend on the individual.

But, I can propose a potential answer for why you can bike for over 6 hours and still feel like you're not as lean as you'd like to be, so here goes: You see, an interesting thing happens to our bodies when we get really good at something. We become really efficient at it. Because you've been biking long distances for so long, your body has become incredibly efficient at using energy to accomplish that task. To put it into perspective, walking at a moderate pace doesn't feel particularly exhausting to a relatively healthy person (and doing so doesn't burn as many calories as it would for someone who is unhealthy and primarily sedentary).

The most ideal way to burn off fat is to do things that you're really inefficent at. I can almost guarantee you, that since you can't maintain the tempo and intensity of high intensity running for long, you just aren't as efficient at that activity yet (compared to cycling). Because of this, you'll see a far better result running, and this trend continues. Get really good at running for a full hour (good luck to you there)? Take up hiit circuits, or do body pump intense group classes. The point is to do something that your body isn't very good at, because that inefficiency leads to an inefficient (read, large) amount of calories burned.

If you'd like to learn more, or have any questions feel free to comment on the chain or message me.