I'm just wondering, if you go above your target heart rate are you burning muscle?

Not necessary: I assume you are talking abou your maximum calculated hr. If thats the case, not necessary. Depend how long you stay there. With high protein diet, weight lifting and short burst 1-2 mins every 5 mins of fast running for no more than 30mins, 3 times a week, probably not. Every person is different but you will built endurance and cardiac capacity. Check with a trainer for better results.