The Time Required
No matter how you slice it, walking takes time. If you're in a big hurry, you'll get much faster results from higher-intensity workouts like running, vigorous cycling or vigorous swimming. If you really want to stick to walking but don't have a lot of time, shifting from a leisurely 2 mph stroll to a brisk 3.5 mph walk will increase your calorie burn by about 50 percent. Taking that 3.5 mph walk uphill, instead of flat ground, increases your calorie burn by more than 50 percent.
You Need Space
In theory, you could get a good workout by walking in circles in your living room or the backyard, but that's a recipe for lots of boredom and stubbed toes. To really enjoy your workouts, you need either a good stretch of space where it's safe to walk -- at least a city block or a mile of trail is enough to start making things interesting -- or a treadmill. Walking outside is also weather-dependent; you'd better be prepared to deal with extremes of heat or cold, plus rain, snow and icy ground depending on the local weather. Finally, if you're walking near a roadside, you'll need to watch out for vehicles. Good safety precautions include wearing a reflective vest or at least a brightly colored jacket, and a headlamp to make yourself more visible at night.
Weight-Bearing Can Be an Issue
Walking generates very little stress on your joints. That makes it an excellent low-impact exercise for most people, including those with joint problems or osteoporosis -- but for those who are morbidly obese or have truly severe joint problems, the weight-bearing aspect of walking can still be too much at first. Exercising in the pool is a good alternative because the water supports your body, taking the stress off your joints and helping build the strength and endurance you'll need to work up to a walking workout.
Pool workouts are also an excellent alternative for people with balance issues, for whom going on a long walk might pose an unacceptable risk of falling. If they can be persuaded to use a balance aid, people with stability issues can also benefit from walking with a cane or walker -- although they may still need supervision due to fall risk.