Well, I don't know exactly how long it takes, I don't check out the clock all day, 30-45 minutes was how long I felt it takes, but maybe I unconsciously took into consideration the time needed to go to the park to do that workout and back. For my core, I do leg raises. What's wrong with them? About stretching, there are different views, even among experts, about its usefulness. I guess it depends on the individual's body. My experience taught me that my only muscles I need to stretch (because I pull otherwise) are my inner thighs.