If you can run a kilometre smoothly, than the physical aspect of it is easy. Basically the hardest part (for me) was staying in sync with the terrible music and the manly voice behind the music. You start out depending on your age, so in example I did it when I was 17, the instructor gave me a warm-up on how it would be like etc, than again depending on your age I assume you start at a stage, I believe recruits 16-35 start at stage 4. Before you start they put a heartbeat monitor thing on your chest with a watch on your wrist that measures your heartbeat. So each stage is 3 minutes long and keeps getting harder every stage, what they do is you start and you keep going until your heartbeat reaches a maximum level. Since im 17 my end point was 173 beats per minute, I lasted to Stage 6, and ended with 193 beats per minute since if your on a stage you cant stop, you must complete every stage from start to finish until the instructor tells you to stop, (Usually at the end of which ever stage your on) SO...the point of all this is the measure your oxygen intake, for me I believe my minimum oxygen intake had to be 39.1 and I finished with 46.7 in w/e units they're in just kinda going off what the guy told me. So along as you can keep in tune with the crappy music + steps and able to at least run a kilometre continuously it should be a piece of cake for you. The real demanding part of it is the strength ie- pushups, grip, sit-ups. Honestly I went into that as like a cocky teenage just like any other thinking it would be a piece of cake, 20 "Combat-Pushups" as i was told thats what they were called are pretty hard, like you wouldnt think they would be but until you actually do 20 consecutive perfect pushups they're pretty physically demanding.