I also through the bs flag on this - I'm down 90 lbs doing just what he is telling you what not to do. I agree with the other posters that it has a lot to do with what you eat before, during and after. You have to maintain your calorie input during the long rides and it has to be the right calorie. I do not believe that all calories are created equal.
There is a very good book call "Ride you way lean" that gets into the science of heart rate as well as endurance cycling. But bottom line is if your heart rate is in zone 4 or 5 for an extended period then yes you will loose some muscle but if you keep it in zone 2 then your body can burn fat stores fast enough to replenish the glycogen stores.