I believe that it is really individual preference and goal orientated.
Yes, running does place an increased amount of impact on joints, especially when someone is carrying a little extra weight. Therefore walking can be a great alternative. However, you have to remember that your body will get used to that 1 hr walk in ~3-4 weeks and you will have to change something up so that you limit or prevent yourself from the infamous plateau. A great way to do this is change your walking routine (if that is the only activity you are participating in). For example, increase walking speed for short durations or do some hills (intervals/hill training)...both will allow you to increase the number of calories burnt...while reducing the possible plateau. Also...could add walking poles....