well.. to a sense i guess he maybe giving you the right information. What type of energy your body is expending during your "cardio" is based on your VO2 which is the amount of oxygen your body needs for the amount of work your putting it through. now its a total myth that you can "burn" muscle, its simply not scientifically sound information.. you can eat away at protein stores while doing intense cardio and your muscle can decrease in size due to lack of needed protein yes but don't think your eating your muscles away. anyway before i get too deep into information... aerobic excersize....50-80 percent of your maximum heartrate is a good template for burning fat stores during excersize, and unless your doing anaerobic excersize wich is like 80-90 percent of your max heart rate and eating no carbs you wont eat away at your protein stores.Now what dictates "too much" is your own mindset and if you love doing it by all means continue... hell professional runners run a little bit more then 2 hours a couple times a week and id say they are in pretty good shape =) i don't think its crazy and if its what you like doing then by all means continue.
good luck with your training!