Exercise for the New Year

A writer's life is a solitary one. It's also a very sedentary one. When I was younger, that was not so much of a problem. But as I get older, I'm finding the need for exercise--not only to keep weight gain off, but also to keep my muscles and joints working the way they're supposed to. 

Which is why one of my New Year's goals has been to make time for exercise every day. While I enjoy walking outdoors with our two golden retrievers, I don't particularly care for mud or a frozen nose. So I found an alternative through Leslie Sansone's Walk at Home exercise program. 
This week marks the start of my fourth week, and I'm enjoying it immensely. I'm starting off slow--one mile a day, but each day concentrates on a different muscle group, and I hope to work my way up to 4-5 miles. What type of exercise do you find effective? What keeps you motivated?