She's saying "I didn't realize what I had in store".
In this context, it means that at the time she didn't realize how much work would be needed to plan a wedding. She could have said, "I didn't realize how much work I would have to do".
Having something in store usually means having something planned for you, or having something waiting for you.
Another example: Before I went to university, I didn't realize how much work I had in store.
I hope this helps.