Q&A: Is Running Safe?
Yep. Running (or other moderate exercise) doesn't seem to have any effect on the quantity or quality of breast milk. Just remember to take care of your breasts by wearing a supportive, well-fitting bra and breastfeeding baby before heading out for a run (breasts that have been emptied of milk are much comfier for running). Be sure to stay well hydrated too, and keep an eye on your weight: If you start losing more than a pound each week, you probably need to add more (nutritious) calories to your diet.
Note: Always get an okay from your doctor before starting an exercise routine postpartum.