Workout after open-heart surgery, bad knees, and at 60 years of age

I need advice and input. Just now healthy enough to exercise, gain strength, regain balance, and tone up. I am 60 years old and I teach at an elementary school. My job requires me to get students, bring them to my room for a reading support lesson, walking them back and then get the next group. I usually see 8 groups a day. I had an electrical heart defect that caused me problems for several years. I received a pacemaker 03.24.2021. Unfortunately there were complications. I had emergency open-heart surgery today on 04.02.2021. I am starting up a brand new school year, so excited to have my life back, my health back, and I want to workout. I am 5’4” and weigh 138 lbs. I have osteoarthritis in my knees and they can give out at times. I have asthma which is under control with meds. I would love to gain strength, tone up, and work on balance problems. I gained fat in my midsection and upper back during the last 2 years. What’s the best way to start and maintain a healthy workout program. What exercises would benefit me the most given my health history. I have hand weights, exercise bands, and a mat.