The fat burning zone is something of a myth. It is calculated to be the zone (heart rate) whereby most of the calories you use are drawn from fat.
It is not the zone whereby you burn the most calories. At higher rates of exercise, you burn more calories, including more fat calories.
I have read that exercising before eating is a bad thing, in that the body senses it is at risk (no food) so draws on glycogen and lean muscle mass while protecting its critical fat reserves. Better to eat say even half a banana, then exercise.
On walk Vs run over the same distance. Looking at the activity calculator here, which I find correlates accurately to the data on my rowing machine, a 4 mile walk at 4 mph burns 424 calories, while a 4 mile jog at 6 mph burns 565calories, so not the same deal at all.
On over-doing cardio: if you kept your heart rate to around 65% maximum (start with 220 - your age in years as your maximum) you would be safe to keep going longer than the OP reports. But you need a heart monitor (around ~$50).
Start weight: 261 lbs, July 2011. BMI 34.5 Obese (91st percentile)
Current weight: 246.8 lbs Sept 6, 2011. BMI 32.6 Obese (84th percentile)
1st Target: 227 lbs <insert date> BMI 29.9 Overweight (71st percentile)
2nd Target: 205 lbs <insert date> BMI 27 Overweight (51st percentile)
Final target: 189 lbs <insert date> BMI 24.9 Normal Weight (32nd percentile)