pushing hard or taking it easy, which burns more?

Every sunday morning, I go running in the mountains. It is pretty steep and we do about 7 or 8 km. I wear a polar hm and normally we take 1,5 hours to arrive and I burn about 500 or 600 cal and about an hour to descend, running almost all the time and burn about 500 on the decent for a total of 1200 or 1300 cal. I normally stop the watch when we get to the top and we probably rest about 10 or 15 min before heading down. This week, because it was freezing and there were some ice patches and we stopped to take some photos, we took it pretty easy and instead of taking 2.5 hours round trip, it took about 3.5 hours to cover the exact same distance. I was shocked when I looked that the polar, 1717 calories! I understand it was another hours time, but at a much lower heart rate. I get up to 165 bpm or more normally and I think I did not break 130 this time. How it is possible?

 1717/3.5=490kcal/hour. 1300/2.5=520kcal/hour.

 Some artefacts of the rest periods as well in there I'm assuming, and are you recording the calories used on the descent or just guessing? Lower output sustained for longer eventually catches up to and surpasses the energy expenditure of more intense exercise performed for shorter duration. Since most people don't have infinite time to work out it's best to work out at an intensity you can only just sustain for your available training time.

Thanks Melkor,

I normally reset the hm at the top, so the decent is the actual figure. I just assumed that covering the same distance, pushing as hard as possible would burn more calories. I normally have to change my shirt at the top and then again at the bottom before getting into the car, the other day I hardly broke a sweat, it just does make sense to me. I just noticed you broke down the calories per hour, I guess that does make sense. Thanks

I am not an expert but did a "tour" with my local gym in their biggest loser contest and learned a lot. The hardest concept to grasp for my engineer teammate was the fact that a lower heart-rate burned more fat calories than a high intensity one.  I find it more satisfying to see sweat and think it's doing more good. As far as cardio workout... it is, but fat burning is done at a lower metabolic workout.  On a physioligical basis, I think you get more steady oxygen, which is necessary to burn anything even calories than you do at a higher intensity.  Know your FAT BURN zone and stay in it as long as you can.  Hope this helps to understand... our bodies are complicated... If they weren't we'd all be in shape... right??