I'd suggest asking your veterinarian what "https://www.vetary.com/dog/condition/heart-murmurs (grade)" of murmur you're dealing with. If she hasn't already suggested surgery in the case of a valve defect, or a diet modification, we can assume the grade isn't very high (I/II).

A veterinarian cardiologist will be able to expand further on exercise restrictions (whether on the phone or in-person), especially if the grade is further up the scale. The majority of low-grade cases call for simple diet changes and less extreme exercise which is then monitored weekly for progression.