fast walking is actually one of the best exercises out there. If done right and at the proper time then it can be a powerful tool. Unlike running your body will not burn muscle, you will however burn fat. Which is what we all want.
Walking first thing in the morning after getting up is very beneficial as that is the time when your body will burn the most fat. It also helps to keep your bone density up and helps you maintain muscle mass. So go for it. 45 mins+ of good walk time = one of the best forms of exercise out there.