According to Mayo Clinic, walking at 3mph burns between 314-469 kcals/hr [1], depending on weight. That's 3140-5628 kcals for 10-12 hours of walking, depending on weight.

Now consider the amount of calories available [2]. I could have the IHOP meal for breakfast, walk for 12 hours (side note: who the heck has time to walk for 12 hours?), not eating anything all day, then eat the Dickie's meal for dinner, and still be well over the kcals burned.

jp555 is right, you can't out-exercise a bad diet - what you eat matters.

[1] http://www.mayoclinic.org/healthy-lifestyle/weight-loss/in-d...

[2] http://news.health.com/2015/06/03/the-9-highest-calorie-meal...

The fat burning zone (I prefer the term "fat burning bias" because it's not a switch but a gradient) would only begin to dominate as you use up all the energy in the food you eat. Any food you eat (not just carbs!!!) will cause insulin to be released, which "smothers" the fat release signaling (just like you wouldnt dip into savings when you have cash on hand). Only when available food energy diminishes, and therefore insulin diminishes does the catecholamine (epinephrine, etc) signal dominate, driving body-fat release for use as metabolic fuel.