I find that the ride you're worried about is the one you are more careful with pacing about and it goes okay.

However, just to air the otehr side here, I'd look at the weather that day. Going on a really long ride that ends up being NO FUN can make you reluctant to go out the next time - there's something to be said for quitting while they (your legs) still want more, to keep the desire up.

(On the third side, though, if you *don't* go, if that will mean you hesitate for the next long ride, too... it could be a 'slippery slope' that way.)