Walking is certainly the main part of my exercise regime...I actually enjoy it.
There are things you can do while walking to increase the diversity of muscles worked out & give yourself a better workout (although a good brisk walk is a great place to start).
I use a book called Walking for Health
by Dr William Bird which has some good stretching exercises and tips for avoiding common injuries such as shin splints as well as good ways to add variety to walking (eg lengthen/shorten your stride for intervals, bring your arms into the mix etc). I'm sure there are websites which have similar information but I've never bothered to look as I found the book first and have found it very useful.
If you walk the dog on a leash you can get an attachment so that you can attach your leash to your belt/waistband so your hands can be free for the proper power walk and you don't get the poor dog all tangled up