Does having a lumpectomy affect my ability to exercise?

No. The recovery following a lumpectomy is on the order of days rather than weeks. Exercise is usually very well tolerated as soon as one week following surgery.
ASK YOUR SURGEON. Too many variables so ask your doctor simple superficial lumpectomy away from the axilla 2 to 4 days.- no heavy lifting no bench presses. Larger, deeper, closer to axilla 7days due to discomfort i allow stair master or jogging with limited arm motion after 7 days but every case is individual. No swimming for a week. You should be golden at 3 weeks - all depends - on your situation!