It's not like you're against the idea of cardio conditioning. You just don't want to risk losing any of the muscle mass you've built hoisting heavy stacks of plates. If thoughts like these weigh heavily on you while running a treadmill, a study published in the Journal of Strength and Conditioning Research offers some helpful advice.
To find an endurance training program that's muscle size and strength gain friendly, scientists examined 21 different studies. They found that strength training by itself or concurrent training (strength + endurance) produced greater results than endurance training alone. For concurrent training, strength + cycling produced significantly better size and strength gains than strength + running. Frequency and duration of exercise negatively impacted size and strength, but had a positive effect on body fat percentage and maximal heart rate. Protect muscle tissue with a pre-workout protein shake, and find a balance that's the best fit for your goals.