There is a DVD with a 1,2 and 3 miles. Some peope feel you can buy a 3 miles and just do up to where you feel comfortable and then fast forward to the cool down. When I started I liked completing the 1 mile workout and I think the pacing is a little different in each segment.
All of that to say if you can find the WATP DVD with 1-3 miles you will have something to grow into.
When I first started, I got the set with the 1, 2 and 3 mile walks on it. When I first started, I could only get half-way through the 1 mile. I just kept at it until my endurance was up a little. Now I can do the 1 mile walk twice a day.