Originally Posted by john m flores

There's a fundamental difference between running and walking in terms of muscle groups engaged. I ran a hilly 50k some years ago and I made one mistake in training. My plan all along had been to power walk some of the uphills, but I had not done much walking during my training. So during the event, when I tried power walking, I found my walking muscles to be lacking and fatiguing quickly. So, quite ironically, I had to run up the hills because I could not walk them.

If the goal is to run a marathon, training by walking won't work all that well.