Because Mitt Romney is younger than John McCain that gives him a clear advantage. Also, McCain was hurt badly in the Vietnam War because of his torture experiences so that doesn't help him. It's easy to beat up an old man, but it is not nice. However, Mitt Romney wins here.
Mitt Romney is far younger than John McCain and because of this Romney wins. McCain is just simply too old to fight on his own. His torture back in the Vietnam War also hurts him because he cannot fight as well as he used to. It's an easy win for Mitt.
McCain is a warhero and survivor is impossible situations, age can remove someone from operations, yet the killer instinct remains (part of why Veterans are considered a danger to the current government).
McCain has anger problems. He won't care if he breaks his hands while pounding Romney's face out of shape.
McCain is actually in better shape than most young people (when campaigning for the presidency, he took some time out to scale the grand canyon with his marine son).
Not unless Mitt gets about three hours of prep time. Without any prep, Mitt would easily be defeated by John. John McCain served in Vietnam, which means he definitely has got some good fighting skills in him. However, John is getting older and that may give him a disadvantage. If I had to choose, I'd say it would be a hard battle but John would eventually prevail.