Hello This is very nice to note down that Cardio-pulmonary stress test with EKG normal i.e. no problem for heart and lungs . Now about your question. MOWING for lawn is a very good exercise . As you mentioned that during this process heart rate increases and after rest of 15 minute , again resume work , till completed . Since all functions are good so need not to worry and this is not a cause of concern. Still I advise high fiber diet , green leafy vegetable fruits. At this age cut down the processed food , white bread ( in limit ) . A 40 minute physical activity like mowing of lawn is good for you ( as you are already doing ). So keep it up , no need of any consultation or concern. Good luck.