Take your age. For example, 26. Now, subtract it from 220. This number, in my case, 194, is known as your Heart Rate Reserve, the maximum number of times your heart can beat per minute before it explodes from your body and kills you, or possibly the point when you become the Incredible Hulk. It's more theoretical than actual science, but bear with me.
Next, measure your Resting Heart Rate. To do this, don't fucking move for a while, you twit, stop it. Now, take your index and middle finger and press it gently to the top of your throat, just where your jaw meets your neck. That pulsing? It's called a pulse. Time out ten seconds, and count how many times you feel that beating. Now multiply that by six. I got 60, or 60 beats per minute. The average adult human falls somewhere between 60-100 bpm, and the lower yours is, the healthier your heart is (or possibly the more Buddhist you are).
So now, take your HRR (The big number) and subtract your Resting Heart Rate (The smaller number). Now, for me, I'm down to 134. Take this number, and multiply it by .65, or 65%.
Still with me? Good. Take your new number, and add back in your Resting Heart Rate. I'm up to 147.1, but fuck decimals, let's round that bitch. 147 being the final number for me, we have arrived at our final destination. This is the rate at which my heart rate needs to beat for me to actually burn fat from my body.
A guy named Karvonen made this formula up, and it's the medically accepted way to figure out heart rates, percentages and other things, and 65% is the approximate rate that the heart has to be working at for about 15 minutes before the fat cells in your body begin to be accessed for energy.
Too much? TL;DR? Run for MORE THAN 15 MINUTES, fatty, and soon you won't be!