I'm looking to join the CAF (Canadian Forces) myself at some point in the future, and the #1 thing I recommend, is you be in ABSOLUTE tip top shape (like, strength and endurance wise, quit smoking, etc) and practice any parts of the Basic Training and movements (turn right, turn left, about turn, marching, etc.) you can. It will help you in the long run, especially during the Obstacle Courses where you'll be expected to lift yourself up over walls and also helping your fellow Squad Members, Team Work is also to be expected.
I know in Basic Training for the CAF they do 3K Marches than 5K Marches as well. This is also done w/ all your gear, approximately 55 pounds worth.
This will naturally be quite different for the US
I suspect.
Try and see if there's any videos detailing what's to be expected.
You will also be yelled/screamed at and generally be tossed around like a pebble in a tin can. You must also have an aptitude to adapt to situations quickly and effectively.
You'll also be subject to a Medical Test as well, and may have to provide a copy of your medical history. (At least I know that's how it is for the CAF, though the US Army is surely similar)
Also,
@WittyAspie was in the US Army, so he may have some more insight for what to expect