I have had this evaluation done and you are right they are trying to get you off LTD. I was on it for years. When they ask you the questions, answer them as if you were having a bad day. Even if you are having a good day. Trust me the PT guy/gal is not "in it" with the LTD people they are just hired to do the the testing.
This thing is going to last about 2-3 hours long so you will get tired and fatigued. It will show up in the testing. They will do dexterity, logical thinking, climbing stairs, walking a certain length of a hallway. Stuff like that. DO NOT PUSH yourself. Do what you would normally do on a normal day. If walking up a flight of stairs is painful to you daily then say so. They will do most things to simulate what a "normal" day is. Not our normal but a "normal's normal".
I had it done in a non-AC room in the middle of summer. No joy in Muddville that day!
If you have any questions let me know.