The Old Science
Bro-science stated that running eats muscle. This was based on studies of elite endurance athletes vs. elite powerlifters. You are neither.
The Interference Effect
Yes, molecular signaling pathways (mTOR vs AMPK) can conflict, but only at high volumes. If you run a marathon and try to max your deadlift the same day, you're gonna have a bad time.
The Heart is a Muscle
You know what kills gains? Dying at 50. Cardio improves capillary density, which helps clear metabolites from muscle tissue between sets. Better cardio = faster recovery = more lifting volume.
The Sweet Spot
Keep impact low. Rucking, cycling, or swimming are superior to pounding pavement if your goal is leg size. 2-3 sessions of Zone 2 cardio a week will actually *help* your gains.
