ive started back up with kickboxing. 40 to 50 mins at the most and it keeps the heart rate up and uses most muscles. i adapt pretty fast to cardio also. i am tired usually before and dont want to do it but when im done the boost after stays for several hours. all the kicks and punches make the workout go fast. i noticed dehydration is what gets me tired. so i have to drink alot before, during and after the kickboxing.