Is Whitebeard a half-giant or a man with giantism? If the latter, I propose his health problems aren't from some battle in the past, or just his age, so much as his size.
As people approach seven feet in height they tend to have many health problems, particularly involving the heart and its ability to pump blood throughout the body. Typically they don't live as long as people of moderate height as a result of the stress and strain this places on them. Now, let's look at Whitebeard. He's…what, twenty feet tall or so? I think I read that somewhere, here.
Considering the amount of blood that means he would have to pump, the fact that he is constantly attended to by nurses makes perfect sense at his age. He would need blood pumped into him and near-constant checks on the strength of his cardiovascular system.
The point I'm trying to make? If that's the case, the fact that he still has that muscular build, can be unattended for a meeting, and is able to fight even briefly is more a testament to his power than even the title of Emperor. I'm assuming his health problems might be what brings his duel to Shanks to a premature end.
In any case, in his younger days his great size likely give him an insane advantage over most foes, but now can be considered a handicap as well if it's what causes his poor health. And finally, if that is the case, it's ironic and impressive (And I think I saw this noted elsewhere) that two of the Emperors are handicapped in some way.
