Power walking is excellent exercise, as Nancy said.
I have been doing it as my main form of cardio for the last three years. This Summer I couldn't run, so I power walked my socks off for several months and it keeps me in the same great shape that doing Cathe cardio regularly does, i.e., when I regularly power walk my usual 6 miles distance on the treadmill, I can do Step Blast, RS, etc, no problem.
You will not get injured doing it. You need to wear running shoes, not walking shoes, because when you power walk you go fast and you need the same flexibility in the shoe for that fast heel-ball-toe transition that runners enjoy with running shoes. The shoes marketed for walking are too rigid.
When you power walk at fast speeds (my top whack goes from 5.5 - 6 mph), you are forcing the body to go as fast as it can without breaking into a run, which it naturally wants to do at those speeds. You are constantly holding it back from adopting a form of movement which would in fact be a more efficient mode of travel, i.e., running!, and as a result you use greater energy to power walk at very fast speeds that you would use to run or jog at the same and slightly faster speeds. I use more energy power walking at 5.5-6 mph for 6 miles that if I ran those same miles at 6.4 mph. (Bear in mind that we all walk and run at different speeds. What is fast for me may not be for you or may not be attainable for you, it depends on your height, leg length, natural speed and movement efficiency/gait mechanics. I'm 5'8" with long legs and I walk fast, always.)
The great thing about living in a hilly area, as I don't in flat old MI, is you can get some great work in for the glutes and hamstrings with all those up-hill fast walks. Excellent stuff! Duplicate it, as Nancy suggests, on the treadmill by whacking up those inclines.
I always find that if I try and power walk with TV on as distraction, I never do so well. Your own choice of up-beat music is always a better motivator. I might give up on a 15% incline at 4.5 mph after 1 minute, but if I am powering through a Madonna dance number (or whoever grabs your fancy), then I will keep going at this pace for several songs in a row. Sometimes it is your feet that keep pace with the music, as in the high inclines, but at other times it is your rhythmic breathing that keeps time with the music, for me this occurs at top speeds. Either way, it works just great!
You don't need huge impact, nor do you need to beat your body into submission in order to exercise that heart and keep the mid-life fat gain at bay. Try it out and let us know how you do, OK?
Clare