I have heard rumors that you only begin to burn fat after 20 minutes of cardiovascular activity with your Maximum heart rate of somewhere between atleast 140bpm and (for me) 198 bpm. I am a fitness fanatic but I have athsma which restrains me from running long distances. I can still get my heart rate between the training zone just not for 20 minutes. I can do most other exercises pretty well. I would just like to know because I would like to define my abs by cutting away excess layers of fat. So is it ok for me to run, take a break and speed walk and then continue to run? Will this help me cut away fat in my abdominal area? Many thanks to whoever answers. 10 points to best answer.