Well this is an athropology question believe it our not. The anwser is based on the design of the human structure and the reason we are currently top of the food chain. Simply you are designed to succeed in exercise. Here is a good video which shows the how "human" became top of the food chain. http://youtu.be/826HMLoiE_o
Simply based is that the human design is set to physically work 10-12hrs a day. So we are designed to perform exercise, therefore by the fact you are "human" you can SUCCEED!
Look forward to help you, do it!