Most of us do not want summer to end but when school starts for the kids in a sense it means the end of summer or at least summer vacation. Dropped off my child and as I pass a Starbucks I see not business people lounging outside but my guess moms. I view the going-back-to-school similar to coming back from vacation and needing time to adjust. The first week is readjusting to life without the kids at home for at least 6 hours.
Just as kids need recess to "exercise" before they return to class, as adults, our "recess" has to be intentional with some form of physical activity. Even 10 minutes will do the body good. However, if you have goals to reach (weight loss, gain muscle strength, etc.) it may require, depending on your fitness level, dedicating more time.
In the mean time enjoy the summer but do not forsake yourself. The kids need you and/or your family and friends.