How To Build Up Muscle

is walking on the tredmill for 45 min as effective as running on the mill for 20 min? is speed or time more important when wanting to do cardio- basically burn fat off my ass! I also take spin class about 1-2xs per week, is that a total body wkout or just legs....any workout tips for cardio that I can do on the tredmill or at home? I tend to bulk up when I exercise and Gain Muscle rather than slim down, tips please, help! Im so short and I want to lean down not be short & bulky!