In order to increase your swimming fitness, you should try and develop your ankle flexibility. The more flexible you feet are, the more they act like flippers and are able to propel you through the water with increased velocity. Perform ankle stretches and point your toes as far as you can.
Walking is a great form of exercise. It's easy on the joints, can be done anywhere and the cost is minimal. Start with a daily, short ten minute walk. Then build your way up to thirty minutes per day. You can also advance to running if you want a greater challenge.