Day 4 - Intense Cardio. Protein - 10-15% of the calories in your diet is all that is needed for muscle growth, and your calorie intake must be more than what you burn. You're knackered because your body is not used to it, you need to slowly build your body into it, do not over do it, make sure to warm up with stretches and cool down and shake off to loosen your muscles - very important to avoid injury and is often over looked. That and your eating habbits don't exactly suit you for having high energy levels throughout the day especially if you're working out. Your body uses protein to build/repair tissue, to make enzymes, hormones, and other chemicals, and is important part of bulding bones, muscles, cartilage, skin, and blood. So while you need to make sure to get some to help build and keep your body in order, you do not need to overdo it unless you're doing some serious building work. You can get all you need from your daily diet.