It is definitely a game of strength, speed, agility, endurance and more.
Depending where you play (forward or backline) you training should be different. Fwds push, run tackle, repeat several times with little rest. Backs on the other hand do alot of nothing, oh, well they watch the Fwds do all the work.
Seriously, muscular and cardiovascular endurance us a must. Meat on the bones will help reduce the injuries, afterall we wear little if any padding. Pads are for bleeding females, not men.
Feel free to email me if you want more (and more serious) info.
In the US it's Rugby season now, find a team in your area and check it out.
Rugger