I wouldn't even categorize myself under "extreme cardio". I've been running since I was 12 and even today the furthest I've ever run at once is 10 miles. Personally, I think that people who claim that cardio kills gains just like to pretend they're very muscular under substantial bodyfat like some professional weightlifters are. The difference is, with professional weightlifters you can tell that without and of their fat they'd still be built like a fuckin' tree. People like my brother on the other hand would probably look like skeletons if you removed their fat.