At 25, you should have no problem with the Rescue Diver course, especially if you're relatively fit. I'm 35 and passed my course a few years ago with no problems. I'm relatively fit but had gained a couple of unwanted extra pounds after my wife had our second child (caring for a newborn binds you to the house more than one would think). Anyway, to prep, I tried to get in as much swimming as possible but ended up doing more hiking in the mountains due to pool availability issues. This helped my endurance in the end. The main thing to remember is to pace yourself when doing any of the Rescue Diver exercises. This is key! I watched some of my classmates exhaust themselves by racing at the beginning of the swim tests only to fail because they had nothing left toward the last few laps. If you remember that this is about endurance more than speed and that it is not a race, you will do fine. Getting familiar with your gear in and out of the water will help. Also, try to pay attention to the gear of your classmates and instructors who will play the victims in the various scenarios. This will help you when you have to provide rescue breaths while towing them and removing gear. Again, this exercise will be all about task loading and pacing yourself. If you're calm and conscious of your actions, you will not waste energy and will do fine. It's also very important to eat well and hydrate, as noted by other SB members here. Lots of potassium from bananas should cut down on leg cramps... Anyway, I found Rescue diver to be one of the most fun scuba courses I've taken so far. Keep it fun and exciting and you should enjoy it as well. If you make the most of it, it should be a turning point in your diving education. It is really enlightening and should give you a whole new perspective on your approach to diving. I hope you have a great time with it! Lots of luck!