Most of the injuries are the feet, ankles and legs. Torn or pulled muscles or torn or pulled ligaments. All of the injuries are from walking on uneven ground at night or from small obstacles from walking at night or day that you did not see.
It can also be from just carrying your buddy using the fireman carry and you pull a calf muscle, always make sure you warm up before any activity.
Also good training would be to carry 20 kilos in a backpack and go on a 5 kilometre walk every day, make sure you walk as fast as you can because marches in the military are at a quick pace and most people have not developed the muscles for this and are not prepared.
Also make sure your boots fit you, don't let any one rush you at stores because this is very important, there a lot of confusion on sizes, the Combat boots are sized in English not USA sizes, most running shoes have both sizes written on the tong, so if you are a 9 US you probably are an 8 or 8 ½ in combat boots, the socks are also in sock sizes not boot size. Also having good kneepads like the other fellow suggested would be a good idea, Mountain Equipment co-op has some great ones, or a good skateboard set. Mountain Equipment co-op has tons of great equipment, I would also suggest getting the small tent pegs for your two-man tent and they have some great Gortex black gaiters that will keep you warm and dry.
If you get kneepads make sure they are black or green so you do not get flak.