Believe me you don't sound like an idiot, just extremely worried. You should have heard some of the things we were saying in Gigio's last few days, and we were 5 grown ups! But he was with us for 19 years and it was hard for us too thinking of not having him around.

I agree with the others, it doesn't sound like it's Rufus time yet. And I think that a pet, just like a human, can be happy even with a little bit of pain. I have a lot of people in my family who suffer from arthritis and sometimes the pain is quite bad but there good days and not so good days. Deb said it perfectly:
The things that will tip you off that the time has come will be things like refusing to eat or drink, uncontrollable bowels, severe pain that absorbs his attention.

Please try not to worry too much (easier said than done I know) and talk about it as much as you like, we are here for you.