It could be about fame--how suddenly things are different for everyone, and then the heat dies down and no one cares anymore--but I'd always interpreted it as being about a relationship. "I can just imagine you and me, running out of steam, going through the motions"...have you ever been in one of those relationships where after awhile you're just together for the sake of saying you're together, and you're just too lazy to break up? There's no actual reason to end it, but neither person really cares all that much anymore. I think that's supported by "I cannot imagine growing old, to have and to hold". He's like..."you know, this really isn't going anywhere, the heat is gone" (or will be soon).
It could be about fame--how suddenly things are different for everyone, and then the heat dies down and no one cares anymore--but I'd always interpreted it as being about a relationship. "I can just imagine you and me, running out of steam, going through the motions"...have you ever been in one of those relationships where after awhile you're just together for the sake of saying you're together, and you're just too lazy to break up? There's no actual reason to end it, but neither person really cares all that much anymore. I think that's supported by "I cannot imagine growing old, to have and to hold". He's like..."you know, this really isn't going anywhere, the heat is gone" (or will be soon).