I think one of the most overlooked methods of staying in shape is good, old fashioned traditional resistance training. A properly supervised and instituted weight training program (machines, barbells, dumbells, etc) can be a huge benefit for staying strong, keep the weight off(!), good aerobic health (yes...try doing 30 minute weight circuit very quickly with no rest!), good for bone density, the immune system, etc. Why so many shy away from it is beyond me.