"Too much cardio burns muscle? Really? A properly fueled and rested muscle will be broken down by the body at what point? Is it after 20 minutes? After 23.7 minutes. 34 minutes, but only on Thursdays?" - quote from AAP.
I think the theory that cardio burns muscle comes from looking at long distance runners who are skinny as **** because they run for ages and eat less protein cos they're not trying to get huge. They also train their muscles differently, slow twitch muscle is what they mainly use which as I understand it, develops by becoming more efficient whereas fast twitch develops by growing more and bigger fibres. Most kickers develop massive thighs not from lifting but from kicking lots, very fast and hard. Obviously they'd be bigger if they lifted weights regularly.....