As far as fitness goes, that's the subject of considerable debate. When I left my Reg Force unit we were doing Crossfit with added ruck marches almost exclusively. This was a VERY modern approach to fitness and hadn't quite caught on yet with other unit's organized PT. Crossfit, for the most part, is done by guys who work out on their own time.
Most infantry units tend to stick to more traditional PT. Usually this is a mix of long runs, long ruck marches, and circuit training. I think, in time, this method will fall by the wayside and Crossfit-type training will become the focus.
If you skim my posting history over the past few months you'll see that I'm pretty outspoken on the issue of PT, and there are some who disagree with me. I think the best you can do is educate yourself on a range of opinions and see for yourself what makes the most sense. Personally, if I were to join the army again knowing what I know now, I would probably prepare like this:
I would do Mark Rippetoe's Starting Strength Program until I was at the level of a "Novice" in strength training.
http://startingstrength.wikia.com/wiki/Starting_Strength_Wiki
This is a pure strength program, not bodybuilding. It focuses exclusively on a few major compound lifts. No running, no rucking, or anything else. I believe that if someone were to take the time to develop a strong strength base before moving to other areas of fitness (work capacity, endurance, ect) they would make faster and safer long term gains. Nothing beats pure strength training for developing ligament strength and bone density, and the compound lifts strengthen muscles in proper proportion to each other. You will get bigger on this program, but not in the same way that a bodybuilder would. The emphasise is on developing strength first and foremost, which requires different training than what you would do when going for size.
Once you've stopped making linear gains on Rippetoe's program, I'd switch to Crossfit.
www.crossfit.com
This is where you become a well-rounded athlete. Crossfit specializes in not-specializing, which is why it's becoming so popular. This is where you start developing stamina by going for runs, increasing muscle endurance with lots of pushups and pullups, increasing strength and power through the Olympic lifts, and developing work capacity through exertion against the clock. Crossfit is, in my opinion, a VERY good system. I'll emphasize though, that you'll do better at Crossfit if you take the time to do Rip's program first.
Once you've got a grip on Crossfit, I'd start alternating days of crossfit.com + crossfitendurance.com and crossfitfootball.com. Crossfit Football is essentially crossfit offshoot that emphasizes short fast sprints and heavy strength training. Crossfit Endurance is a supplement to the Crossfit main site and focuses on training for stamina. These two programs, used together, would provide a VERY well rounded military athlete. When it's winter and too cold to run outside, I'd probably leave the Crossfit for a bit and go back to a novice strength training program. In my mind, strength training really is that important. And I hate the cold.
I would not worry about rucking until you're actually in the army. When you are, once a week is probably good enough. Most guys have never lifted a rucksack prior to basic training and we all get through it. While it's important once you're in (especially in infantry units), you're better off using your time beforehand getting in good all around physical condition. Rucking is about conditioning the body and the mind more than anything else. Having a high level of general fitness goes a long way in making it easier.
If I posted the above on the open forum I'm sure some would agree with it, while others would critique the hell out of it. Each person is different and you can't really define the "right" way. What I've written is just a reflection of my own experience doing the army guy job for a little while, and the result of long arguments with guys who also think they have the "best" way.
It's not gospel, but I think it will give you a good place to get started in your own research.
Good luck
-WB