The best way to prevent heart disease, diabetes, cancer, dementia and many other illnesses is free and readily available — it’s exercise. That was the message from Nina Brojan (inset), an exercise physiologist at Memorial Sloan Kettering Cancer Center in Manhattan, at the UFT Welfare Fund Medical Learning Series’ last seminar of the school year at union headquarters in Manhattan on May 17. Not only is exercise “the best drug there is,” Brojan said, but it also leads to instant results. A single bout of moderate to vigorous exercise, she said, can lead to improved sleep, a reduction in anxiety, improvement in cognition and insulin sensitivity and a reduction in blood pressure. “Improvements only become greater with time,” she told the more than 50 UFT members who attended. Walking briskly for as little as 30 minutes a day reduces the risk of developing Type 2 diabetes by 30 percent and physical activity has been shown to lower the risk of clinical depression and of breast, endometrial, colon and prostate cancers. To stay motivated and make sure you exercise, Brojan suggested making it fun and social — choose a form of exercise you enjoy and pair up with someone or join a group. Competition also can be motivating, and remember to track your progress, Brojan said. Sandra DeRosa, a recently retired teacher, said she came out because, “I want to play a part in staying healthy for my retirement, and I want to take charge of my health.” Ellen Mitchell, a teacher at PS 44 on Staten Island, said she appreciated learning “tips to get me moving and help reduce stress.”