It is important that everyone be physically fit, and even more so for a Prepper. Being reasonably fit means that you can hike the trail you intend to travel when bugging-out on foot. It means you can work harder throughout the day to earn a few bucks when jobs are scarce. It means your garden won't wither away and die due to lack of energy.
If you take nothing else out of this, just get up and start moving…begin with a used weight bench and some dumbbells. Your muscles are going to scream and hurt for the first few weeks, but as you continue and see your body change from fat to fit you will want to work out regularly.