Paul, I think you would be suprised about the walking.

Fat burning exercise for a 25 year old is commonly considered to occur between 108 and 128 BPM (heart beats per minute).

A brisk walk should get you in that zone.

Exceed 128 BPM and you are in cardio vascular exercise rather than fat burning.

There is a top to this of course too, which is the limit of aerobic exercise.