As for people in it, you want people with cardio. Cena is a great choice for it as he's in incredible shape and never gets tired. it seems. As for an opponent, hard to say. That's the tricky part really as it's hard to pick someone with enough energy and stamina to last that long. Hard to say who could go against him but I think Cena is a must in it.
I agree with the first paragraph, so I won't respond to it. However,I do believe that cardio is a secondary issue to the problem with having an Ironman match. A bigger issue is how to go an hour without sending the audience to sleep. It can be done, and it regularly did happen in the 80s. Of course, since then the wrestling world has changed, with faster paced matches being the norm. However the pressing issue still remains the psychology of a match rather than the cardiovascular conditioning of the participents.