It'll be alright!
Thundercat, I had exactly the same anxious feelings as you are having now when I went for my initial 3 years ago and the one thing that helped me get through it was the attitude of the nurses.
I'd heard it time and time again on pprune before I went down there. "The nurses want you to pass. They're nice as pie...."
And do you know what? They were!
The lung test was my biggest hurdle, albeit a psychological one, as I used to have asthma, but the nurse had such a relaxing tone and calm manner about her that she hepled me to relax and low and behold I had nothing to worry about.
Just remember, they want you to pass. They're not there to instantly fail you.
Go for it!