What do you do for exercise? Go for a run or a walk with the dog? How about a game of tennis or golf? Maybe you garden, do yoga or a Zumba class? Most of us know that exercise is good for us and that we should do some each day but our busy lives can easily get in the way. Now scientists are discovering just how important exercise is to our minds, bodies and souls. It not only helps our hearts and waistlines, but makes us smarter and happier and our cells younger. It can also help ward off diseases like diabetes and Alzheimer’s. This hour, a conversation on the science of exercise and the dos and don’ts of working out. Marty talks with New York Times “Phys Ed” columnist GRETCHEN REYNOLDS about her new book, “The First 20 Minutes: Surprising Science Reveals How We Can: Exercise Better, Train Smarter, Live Longer,” and with MARTIN GIBALA, a professor and chair of the Department of Kinesiology at McMaster University in Hamilton, Ontario.