If you're just starting to exercise and not quite in shape yet, walking is the easiest less strenous way to begin. Everyone knows how to do it. Once you get fitter, ramp up the speed and distance of your walks. Even during hectic work days, walking is the simplest way to get some exercise, your body needs to move daily. You can do it during your lunch hour.
If you have a lot of weight to lose, I recommend a good fitness program that includes strength training with weights. You can still walk daily or on your light or non-workout days. As with anything, diet is the most important thing. You can walk 10 miles a day and still gain weight if you don't change your eating habits to that of natural, whole, unprocessed foods. Try this healthy fat loss program if you're one who needs a plan or some guidance.