Weight training is a must. If you’re not resistance training while you’re going all-out on your cardio routine, you are going to lose muscle. The more muscle mass we have in our bodies translates to more calories burned, even when we’re not exercising. If you’re just doing cardio, you can lose about one pound of muscle for every four pounds of fat, which means you’re basically losing your metabolism. To maintain your muscle, incorporate two or three days of strength training — think: lifting dumbbells, using resistance tubing, body weight exercises like squats, planks and pushups — into your weekly mix.