This body doesn't do exercise

I used to love to exercise when I was smaller: played basketball, loved bicycling, hiking, swimming, etc.

Now(confession )I take the car around the corner at the apartment complex to get the mail. The doctor says walking, swimming, and water aerobics are all a good way to go. And they all sound like fun, but I just cannot seem to get going. Every once in a while I get the urge to take a brief walk--especially if I drive out of the city to the mountains around here--but once is enough, and then I find that 2 months later I've not done another thing that is really exercise

Any suggestions or words of encouragement--I don't want to wake up one day and find I'm a permanent couch potato