Reese Witherspoon, Heidi Klum, Jessica Biel

Why do ladies on the red carpet always look so good? Because they work at it!

Heidi's run begins on Monday, and we hope you're planning to throw on your sneakers, log your miles and get fit for the next few weeks. In case you need just one last push, here are some of your favorite starlets who aren't afraid to break a sweat.