There is definitely some truth to endurance training having an impact on muscle, so I'll break it down for you.
If you are lifting on a regular basis long distance running will for sure have an impact on your gains. To explain why, the reason is because of the huge degree of damage that your muscles would undergo, so you'd be in a state of never recovering fully, being weaker in the gym, etc. which of course leads to muscle loss.
With that said, running a mile or two a couple times per week really shouldn't be that big of a deal. The biggest issue would be scheduling your lifting and running workouts so that they don't negatively impact one another.
For me personally, I do a full body lifting routine twice per week with some really intense lower body work that would render running the next day a very poor choice. I do tons of sled pushes and hip work so those muscles are pretty much dead for a couple days afterwards. What does work for me would be hiking on a hilly terrain, you are able to get your heart rate up while keeping the impact on your body low. Swimming and cycling may also be a good choice. I think my best advice for whatever you choose to do would be to keep it at a relatively low to moderate intensity.
Also, cardio is good for you but anaerobic training (lifting, etc.) and diet are really the biggest factors in regards to keeping lean.