About six months ago, I submitted a paper for publication. The paper is somewhat long and technical, and I believe that only a small number of researchers will bother reading it (nevertheless, I do believe that my results are nice). I still haven't heard from the referee, but I tend to believe that the paper will be accepted for publication.
A few days ago, I observed that the main results from my paper provide a completely new proof of an old classical result in my field (actually, they provide a new and significant extension of that old result). The proof of this observation is only one paragraph long.
My question: How should I publish this new observation? I can add it to my old paper and resubmit it, but even if it's not too late to do so, I'm afraid that my result won't get noticed (as the long paper will only attract a small number of specialists). On the other hand, uploading a one page long paper to arxiv might make me look funny, and I don't know if any reasonable journal will consider publishing that ridiculously short note.
Relevant biographical information: I'm currently a postdoc. I want to get a decent TT job, so (sadly) it's kind of important for me to impress people in my field.