Try doing a max heart rate test and then check your resting heart rate. You can take the resting away from the maximum and you get you heart rate reserve (HRR). This is a much more accurate way to figure out where you need to be. Percentages can be worked out using the HRR - google it
It really important you dont go too hard to often as you end up knackered. Longer runs need to be easier, that way you recover faster and can do other relevant training like tempo runs.