If you are allowed a preference take the part you are most comfortable with first.
When I did mine I chose to do the Nav first as I was happiest with that, along with the fact that there was hardly any wind on the day.
The nav exercise including the diversion went almost as per the text book, which put me totally at easy and happily sailed through the upper air work and the circuts
I agree with the comments above learn the safety and emergency routines and don't bluff.
Mine lasted about 2 1/2 hours in total and by the end I was drained. It may be worthwhile taking a bottle of water with you as at one point I found my mouth had gone quite dry making the radio calls interesting.
Best of luck when you do it and remember all the training that your instructor has put in has got you to the stage that he or she is confident that you are ready. The instructors at my club did not like their students failing so they made sure you were ready before putting you forward.