The amount of physical and mental endurance needed to be successful in the rescue course can be (unfortunately) dependent on the instructor. In general though, for the physical aspect, 50 meter suicide sprints while carrying a 50 lbs. bag wouldn't hurt. If you have never carried a person before, you will find them shockingly heavy and awkward. Any sprint type exercise while carrying a load will help. If you are in decent shape you will be fine. Regarding the mental aspect, if you have been in a career or a situation that involves task loading, snap decision making (under stress), and rudimentary leadership before you should be alright. If your idea of stress is waiting too long for your latte at Starbucks, you might need to work on slowing down your mind and taking things one step at a time. Slow is smooth and smooth is fast.
Other posters may want to flaunt how awesome they are by pointing out how easy every diving course they have ever taken is but its far more constructive to point out things that may be challenging. In reality, any experienced diver should know that skills that are easy for some can be extremely challenging for others.
Just listen to your instructor, study the course material, and give it a genuine effort and you'll do fine.
-Gabe