A quick post to highlight cycling as a form of exercise (I noticed the walking thread), cycling is an excellent exercise form of exercise and can be enjoyed by people of all ages.
It is low impact, great for strength and stamina, easy to do, good for your heart health and has varying degrees of intensity according to your ability and ambition. These range from a leisurely cycle along country lanes to entering the Tour de Force (who knows, where it'll take you).
For those less adventurous outdoorsy people, there is also Spinning classes. (sorry, I've just realised the forum is 'The great outdoors')