This. Getting into medical school isn't a race, do right by yourself an make sure you are as ready as can be. I also delayed taking the MCAT out of fears that I wasn't ready. It all worked out in the end for me and I will be starting medical school in the Fall. I fully believe it will work out for you, too. Just don't mess up your GPA and keep up the great work!
That is totally understandable and normal. They have heard of students doing this a million times and it doesn't sound like a big deal. Just explain that you weren't completely ready so you rescheduled the MCAT thus had to withdraw the application. I don't think they will really care that much after that.