Although walking is certainly easier on joints than many high impact activities; it is not totally without problems. Walking tends to over use certain muscles, while not using others. If you are adding mileage, speed, or hills be sure to do it slowly. (A general rule of thumb is 10% per week.) To much too soon will more than likely result in an overuse injury.
To avoid problematic knees:
Overpronation is when your foot rolls in excessively after landing, and continues to roll when it should be pushing off. This twists the foot, shin and knee and can cause pain in all those areas. If you overpronate, your shoes may show excessive wear on the inner side, and they'll tilt inward if you place them on a flat surface.
If you overpronate is it important to wear shoes with motion-control and stability features that limit pronation. For some people over-the-counter orthotics or arch supports can help. Those with severe overpronation problems may need to have custom fitted orthotics.
Overpronation causes extra stress and tightness to the muscles, so do a little extra stretching.
Choosing a good walking surface is important to avoid injury. The best surfaces are flat, firm, and not too hard. Avoid concrete if at all possible. This is the hardest walking surface. Hills and cambered roads can also be a source of high stress on joints.
If walking on a road with an obvious camber it is a good idea to walk out and back on the same side. A beach or track with a slant has the same issues. Change your direction frequently to avoid too much pressure on one leg.
If you walk on a harder surface out of necessity, be sure to wear well cushioned shoes. If walking on a rocky natural terrain choose a good hiking boot.
Runner's World ranked the following surfaces on a scale of 1 (awful) to 10 (best):
Continued - Exercises for leg strength and flexibility