Gretchen, here are two other suggestions. First, check out the reviews from earlier tours. Second, just call ETBD and ask. ; ) I did it when I took the Berlin, Prague and Vienna tour. I was getting to Berlin early on and had similar questions.
The castle can be absorb a lot of time if you love history. There is a short guided tour, but then you can do an audio tour that can last hours. The views are wonderful, but you also can see the Honors of Scotland, the room where Mary Queen of Scots gave birth, St. Margaret's Chapel is lovely, but it's hard to find it when its not packed. There some dungeony'prison type areas to explore. There's a great hall where there was a lovely musical program going on the last time I visited. It's also a memorial for the Scottish soldiers who died in war. I find the memorial very moving.
I visit the castle every time I go to Edinburgh.
Pam