Be confident! I took my oldest out of public school in fifth grade and knew nothing about homeschooling. Since I had to make a quit shift early in the year, with no planning, I chose Calvert, just to move quickly into it.
It is the best thing we ever did. I also found that my son, though very strong in verbal and reading skills, had had very little direct grammar instruction. For that reason we switched to Rod and Staff Grammar for 6,7,8. It is not exciting but very thorough and complete and simple to teach.
We used a variety of writing programs over the years--the very simple direct, composition in Calvert for 5th (thorough and adequate), IEW for two years and then Bravewriter for a breath of fresh air from the structure of IEW (which was great for a while but we needed balance).
So all that to say--fifth grade is a fine place to leap. There are a variety of good options, so don't feel overwhelmed that you are looking for the "perfect" curriculum option.
Don't be overwhelmed by WTM. Read it and take some suggestions. Don't try to do it all. School for a while and then pick it up again in three months. Then it will make more sense, and in reality, you will see how much you have already done. Then you will see how some of the many curriculum options out there have helped you accomplish it. Some things don't have to be WTM style. Everyone needs high standards, a big overview of what a smart education is (WTM provides in fabulous scope and detail), curriculum options and then the real world practicality of their child. And then it works.
It really does. It will be fine!:)