For me, the biggest thing was running. Specifically, running as fast as you can, every where you go, including the pool days. A "swim day" will involve running at a break neck pace, doing the pool workout, and running that same pace back, with some calisthenics to finish it all off. The runs burnt out my legs for the rucks and I paid for it.
Swimming and pullups, I did good at. We had at least a few people who got maybe 7 pullups because they didn't do exactly what the order on proper pullups say to do. Swimming; everybody says you have to swim, but its really more like "pool stuff". Go find a MCIWS, ask him/her what "water aerobics" are, and be prepared to do that in full cammies.
The prep guides harp on how the swim test is breast and side stroke only, but I think you wind up freestyle sprinting 500 meters or more. Make sure you can brick tread, AND tow a brick, with the back and side stroke. Thats one of those things they'll drop you for if you can't do it.
By the time you get to a timed event, you're pre exhausted from all the stuff you've done the days before. Your 19 minute 3 mile will turn into a 21 minute 3 mile. I saw the average run/ruck times for a different class, and I shot for those, not knowing this. So my 21 minute 3 mile turned to almost a 23 minute 3 mile, and i wasnt the only one in this boat.
Id say knowing what I know now, make sure you can run a 300 PFT and CFT while you have the flu, go through advanced water survival if your unit has a class coming up, and ruck using the threads on this site for pointers.