I have to defend rugby here, I played for 11 years on and off until my back surgery. I was also forced to play soccer (I used to ski race and for some stupid reason skiers think soccer is good dry land training for racing). Soccer demands a lot of skill and aerobic capacity. It's a sport. But it's in no way as physically as demanding as rugby. I played second row and loose forward. We do all the running the soccer guys do, but as a forward I was rucking or mauling constantly (for those that don't know that's when you drive the opponents off a tackle). Whenever there's a set piece the forwards are either scrumming (the big huddle thing where 8 guys from each team push as hard as possible against each other), lifting or driving a lineout (picking each other up then pushing till you have an anneurism against the other team), or running downfield to do more tackling, rucking and mauling.
Training for rugby is exactly like Marine Corps PT. It takes all the aerobic fitness of being on the track team plus the strength of wrestling. It's about the whole package, not just being able to run.