From Health magazine
You can burn 65 percent more calories simply by cutting your mile-walking time from 17 minutes to 13 minutes—and a strong core can make it easy to walk that distance in less time. Heres a core-strengthening move from Dixie Stanforth, MS, an American Council on Exercise spokeswoman who teaches kinesiology at the University of Texas at Austin; do this exercise two to three times a week.
Side plank
(for core, upper back, shoulders)
Lie on your left side with your body in a straight line from head to toe, right foot on top of left foot, left forearm on floor for support, and right arm extended toward ceiling. Contract abs and lift hips upward; hold 10–30 seconds. Lower, rest a few seconds; then repeat on right side to complete 1 rep. Work up to 3 reps. (To make the move easier, bend your legs.)