I have found that I needed to increase the time I walk for my body to adapt. There needs to be a little resistance. Like I was doing 40 minutes but pushed it to 45 minutes for a couple of weeks. Then some rest days then I was at a higher level of fitness and 45 minutes was as easy as 40 minutes. Don't go crazy. Even 20 minutes most days is great regardless and you get the "training effect"!