Exercise maintains muscle tone, benefits brain fitness, raises endorphins (feel-good chemicals), and oxygenates the brain. Things which involve co-ordination like dancing, playing games, aqua aerobics help to strengthen the synaptic connections used to work muscles.
The trick is to make exercise something you WANT to do rather than OUGHT TO DO. Find something you enjoy doing. Aerobic exercise raises the pulse rate and helps maintain cardiovascular fitness, 20-30 minute 3-5 times a week is an effective level.