You only have to worry about your cardio being catabolic if you are glycogen depleted (like first thing in the morning on an empty stomach). If you've been eating food & carbs, you have glycogen in your muscles & liver & you had that to fuel your run, not your lean body mass.
The reason running is more *likely* to be catabolic (relative to lower intensity cardio) is that b/c it is more intense - it is burning more cals overall & takes more energy -> more likely to get into burning LBM (Catabolism), but again, if you've fueled it with food, you're fine.
Or so I've been told by exercise physiologists.