RE: Katrid - listen to Donna
Although I do not have the Slow and Heavy series, I know enough about it to know it can be an invaluable addition to your workout program.
Far too many exercisers, especially women exercisers, are terrified of "bulking up" through strength training, and overrely on "endurance" oriented muscle conditioning. To paraphrase Donna, when you build muscle you burn more calories, and even more importantly you become more functionally fit.
Without strong substantial muscles there is no effective cardio or muscle endurance work. One of the ironies of human physiology is that strength training will assist in cardio and muscular endurance training, BUT cardio/endurance training will NOT assist in strengthening! Not fair, but that's the way it is.
Annette