I've found that classes are best for me. A few years ago I started going to an aerobics class that emphasized strength. Then I did a boot camp. In 2010 I started doing Crossfit and hope to do it the rest of my life. They push me well out of my comfort zone, and the group aspect makes it both competitive and collegial.

Last year I decided two times a week was good enough for me and it's worked. I'm in fantastic shape.