Start slowly and try to build your walking regime gradually. To get the health benefits from walking, it needs to be of moderate-intensity aerobic activity. In other words, it needs to be faster than a stroll.
Moderate-intensity aerobic activity means you’re walking fast enough to raise your heart rate and break a sweat. One way to tell is that you’ll be able to talk, but not sing the words to your favourite song.
Source : nhs.uk