From this article this quote: Exercise: The News You Don't Want to Hear
“A much better and more efficient way to exercise -- and one that research is clearly showing works a lot better -- is to do high-intensity circuit training. Put the beauty bells down and lift some iron. Shorten your rest periods. If you're doing "aerobics," do some interval training where you sprint for a while then jog to catch your breath.
And by the way, forget about "toning." It doesn't exist. You're either building muscle, maintaining the muscle you already have, or your muscles are slowly shrinking. The first two are accomplished with weight that's heavy to lift. The third is accomplished by doing nothing.