Is cardio going to undo your strength training?

Not at realistic volumes. The interference effect is real but it shows up mainly with high volumes of hard endurance work alongside serious strength training — think several long hard sessions a week. Two or three moderate cardio sessions will not cost a recreational lifter meaningful muscle, and the cardiovascular benefits are substantial and independent. If you want to minimise any overlap: separate them by several hours where possible, put the priority first in the session, and use lower-impact modalities like cycling for the legs. Most people do far too little cardio, not too much.