You will see improvements if you consistently follow the plan for at least three days per week and greater improvements if you do it four to five days per week. We recommend at least two days of rest or cross training per week. The days off from running give your legs a chance to recover, which you need.
If you are like most people, at some point you are going to miss more running time then you planned. Life happens. It is imperative that you know this - when you miss more than a few days of running, you are going to lose some level of fitness. And you will lose that fitness quicker than you gained it. That's the bad news.
The good news is that you can get that fitness back quickly. It won't be like starting over.
First and foremost, accept that you have lost some fitness. For example, if you were running 2 miles comfortably and then missed a week, don't expect that you can get right back to running that distance with the same degree of comfort. When you miss some time, remember the basic premise of this program. Start with a shorter distance and walk when you need to. Don't feel bad that you have taken a step back. Your physiology still has changed from your previous work and you will get it back.
Keep in mind that the more you missed, the more fitness you will lose. But we'll say it again, don't let this bother you. Remember why you are doing this - better fitness, weight loss or whatever motivated you to begin with.