Your ability to walk at a certain time can depend on your stature as well. 4mph is considered fast walking...but it is going to be much fast for someone who is 5'1" than it is for someone who is 6 feet tall.
Take that into consideration as well. When my husband and I walk together, he I counted that he takes 2 steps for every 3 that I take. My legs are shorter than his.
What is important, is not simply the time or the distance, but that YOU improve over time. I think distance is the thing, really, to start with. Many people when they start, can only walk 1/2 or 1 mile-especially if they are very sedentary or very overweight. They slowly progress to being able to walk 2 miles, 3 miles, or whatever.
At the same time, you will also over time become a faster, more fit walker.
So...I just think that over time you should slowly start increasing your walk, and become faster as well. In the beginning you might be walking 1 mile in a half hour, and later on, you might be fit enough to cover 2 miles in that SAME 30 minutes. So, you are still working out the same amount of time, but you are covering much more distance.