Well would you rather have muscle and be lean and toned or flabby. The older you get, the more gravity will pull everything down. If you start now and keep in shape, you will have a lot less problems in the future and be able to maintain your weigh easier. Walking/jogging will keep your muscles lean. Lunges keep you toned. Unless your start adding heavy weights, I doubt you would bulk up at all. Being strong and healthy is the best feeling in the world. Your off to a great start!