Sometimes your body plays tricks on you...
your legs get smaller, less full, less cuts... so you think it's negatively impacting your prep...
stop cardio for 2 days and load up on carbs and sodium.. if on the third day your legs look bigger than they ever were and totally pumped, then it's good for you. if not, then i guess that kind of intensity on cardio isn't good for you..
i'd say stepmill or "the gauntlet" would kill your legs if you are not the type of guy who gets the best leg training by doing higher reps and lower weights. switch to something else that keeps your heart rate up, but doesn't fatigue your muscles too much. YOU SHOULD NOT BE TOO TIRED WHERE YOU CAN'T TRAIN LEGS AT FULL FORCE!!
my legs and calves ALWAYS grow when i do more cardio... they get smaller while i do the cardio, but once i stop for 2 or 3 days and start eating more "junk"
, they look bigger than they ever were.
but, as most have said, it's trial and error bro... but cardio is just that... CARDIO... you shouldn't tire out your muscles from cardio.