Starting weight training has cost me my cardio gains?

To some extent, yes. Physical fitness is not one homogeneous thing. Fitness can be surprisingly activity specific . . . and yes, if we don't continue to challenge some capability, we won't progess; and if we reduce our training of the capability, it will decline. I'd expect cardiovascular capability to decline if I went from 5 days of cardiovascular exercise to 2.

It's more complicated than that, but that's the oversimplified version. For example, I don't see a lot of cardiovascular capability loss in the Winter when my workouts are shorter than in Summer (because machine rowing isn't as fun as on-water rowing, still oversimplifying). But I tend to increase the frequency of more-intense CV workouts in Winter, and there's some tradeoff between intensity and duration.

I'm not suggesting that you increase your running intensity, because - now that you presumably have strength goals - ramping up the cardio intensity might detract somewhat from your overall recovery. It's complicated, and somewhat individual - I suspect - besides.

Have you talked with your trainer about how to handle this? That'd be where I'd suggest you start.


ETA P.S. Maybe think of it this way: Let's say you're a musician, a pianist, and you practice every day. Then you decide to take up the guitar, but you can't allocate more total time to music, so you cut back piano practice to a couple of times a week, and spend most of your time on guitar practice. You'd expect your piano skills to get just a bit rusty in that scenario, wouldn't you, even though both are music? This isn't really different.

Already explained above.

One more thing though. You can get back the previous aerobic performance quickly, much faster than the rate you lose it. If it's important to you, you should probably periodize, so every so often return your focus to running while still doing some weights, then vice versa.