Carrie Underwood's latest priority is longevity. Joy Malone/Getty Images Singer-songwriter Carrie Underwood prioritizes longevity with her routine. She said her habits include working in her garden, a form of exercise that supports healthy aging. She also swears by doing push-ups in every workout and getting more protein to build muscle. When Carrie Underwood grows up, she wants to be a fit grandma. Now in her 40s, she's looking ahead to the coming decades and dialing in her longevity routine. The country music star, known for stage shows so energetic that she prepares by singing on the treadmill , said her life's goal is to work on a farm well into her golden years. "The decisions that I'm making today are going to determine what my future looks like 20 or 30 plus years down the road," Underwood told Business Insider.…