Walking backward is touted as having many benefits in an opinion paper by Barry T. Bates, B.S.E., Ph.D. and Janet S. Dufek, Ph.D., FACSM. They studied backward walking and running in their laboratory at the University of Oregon. They concluded that backward walking and running improved cardiovascular function, improved muscle balance, and facilitated neuro-muscular function and balance and proprioception. This paper doesn't appear to have been published in a peer-reviewed journal.
Care must be taken when adding backward walking to your walking program. You need to ensure safety by practicing backward walking in an area free of tripping obstacles. If practicing it on a treadmill, start at a very slow speed such as one mile per hour and be ready to hit the emergency stop. As you become more proficient, you can increase the speed and incline.
Walking and Jogging Backward on the Treadmill
Other ways to add a higher intensity interval to your walking workout include stairs, hills, doing step-ups, and bursts of running or walking at your top speed.
Adding Intensity to Your Walking Workout
Hooper TL, Dunn DM, Props JE, Bruce BA, Sawyer SF, Daniel JA. "The effects of graded forward and backward walking on heart rate and oxygen consumption." J Orthop Sports Phys Ther. 2004 Feb;34(2):65-71.
Bates BT, Dufek JS. "Backward Running: Benefits" Accessed 5/30/2012.