I think walking, along with a decent diet, is effective for fat loss. Barring my teen years, I was at my fittest as far as looks when I was walking at least an hour a day pushing a heavy, triplet stroller.
I lost around 8 pounds doing Basic Step in Jan and hit a plateau in Feb of this year. So, I changed things up and added in more brisk walking, interval walking/jogging and Barre workouts in March. I don't know if it's from the walking, but I've lost a pound already and my thighs have slimmed down a quarter of an inch. Finally! It's definetly not from any diet changes. (My diet isn't great, but it's not terrible.) Now, when I do the interval walking, the speed doesn't increase very much. Also, I don't do a lot of intervals. It's maybe 10 to 15 minutes of 2 to 3 minutes of jogging bursts. The rest of the time on the treadmill is brisk walking. I've found that I can walk at higher speeds without having to jog than when I first started walking on my treadmill. If I do brisk walking only, I'll stay on the treadmill longer.
I also want to add that if your body can't handle stepping, don't step. Focus on what your body can do safely instead. My knees can't handle a lot of high impact step or even low impact stepping every day.